Probably the day my first (and so far only) child was lying in the incubator in the Neonatal Intensive Care Unit with a million tubes and wires attached to him after having been delivered 6 weeks premature.
Knowing that another human being, especially one so helpless, is depending on you is very sobering.
I realized I was an adult when my parents died and after the family home that I grew up in was sold. It made me realize that there’s no more ‘going home’. If I screwed up, no one would be there to bail me out of trouble. Whatever I did or didn’t do, there was absolutely positively no safety-net: Succeed or beg for quarters outside the PATH sation, it was up to me.
Just a word of caution if it’s hip surgery: make sure the blood thinners continue AFTER she leaves the hospital and goes to rehabilitation.
I really felt like a grown-up the first time I filled out the long 1040 tax form with itemized deductions.
My dad died when I was 18. Shortly after, my mother called me at college to talk about the money. My dad had left a reasonable amount for her, and she wanted to know what she should do with it (where to invest it, etc.). I remember thinking, “You’re asking me? I’m a kid!” and then realizing that I had to stop thinking that way.
And recently, I’m remember that moment a lot. We’re slowly drifting into the place where the kids are parenting the parent, and it’s mostly on my brother and me (our other siblings are far away, or have their own problems). It scares me; I’m not sure I’m ready for that responsibility. Ready or not, here it comes.
It’s hard to say when one feels like an adult–most of my life past the age of 18 I just felt like a slightly old child. Maybe it was the first time I mowed my lawn or got real furniture as opposed to milk crates and cable spools.
Maybe it was when people started to take my opinions seriously instead of regarding me as just another silly kid. (This was roughly about the time I began writing off people younger than me as silly kids.)
Maybe it was when I realized that most of the music I like is 30 or 40 years old and that I don’t much care for the music of today.
Possible it was when I realized that I have some problems and that while people can help me out no one can just wave a wand and solve them all–ultimately I’m the one that has to deal with them.
Now meeting people who can’t even remember the cold war—that just makes me feel old.
I think becoming an adult is kind of like one long process where the immature bits of you get chipped away over time… I ‘‘felt’’ like an adult when I was supporting myself at age 17, but emotionally I was still a child in many ways. Every so often I have this ‘‘aha’’ moment of adulthood… ‘‘Aha! Up until this point I thought X was a reasonable course of action, but now I see it’s not. Here is one more step on the journey.’’
But I admit I did feel a sense of smug grown-upness when I started contributing to a 403b this year. Most people I’ve discussed this with are shocked that I’m saving for retirement at age 24, but that 10 years can make all the difference in the world. I think ultimately adulthood has to do with foregoing the immediate pleasure in the interests of the long-term health and stability of you and your family. I have to believe that is a lifetime learning process.
I’m a department director at a college, and I started working as soon as I finished college. In the beginning years, many of the graduate students whom I advised were older than me.
(A few ahem years go by …) Then one day a young person came into my office and asked if I could help her with a project that wasn’t going well. Sure, have a seat, let’s work it out! Then she said “I’m a professor in the Biology Department, and I’m having trouble with blah blah blah …” In my mind, my first thought was “You’re a PROFESSOR?!? Why are YOU asking ME then?” I was seriously amazed, and then the pieces fell into place – I was in fact a gainfully employed adult who was viewed as having expertise in my field, and authority to make things happen at a policy level.
Whoa.
NPR-check
Gray hair- check
House- check
Parents died in my 20’s-check
It was this year when in April I decided to quit my good paying job I hated. I spent years building up the skills and knowledge for that job, It wasn’t any damn fun, gave me no reason to get up in the morning.
I took the summer off and went to Ireland and the Isle of Man for two weeks and had a blast. I’m now working in a small machine shop with a friend making a third of what I was and having a blast. I’m going to build this shop up into something I can call my own, with my own minions.
My adulthood was realizing I can do whatever the hell I want after all.
I realized I was becoming an adult the first time I turned off a light that wasn’t needed because, damnit, I had to pay the bill. Just like my dad used to do! When he was really on a roll, he would walk through the house indiscriminatingly turning off lights.
“Let’s turn some lights off!” Snap.
Voice from darkness “Hey I’m in here!”
I also noticed that people only drinking half a can of soda and then abandoning it really chapped my ass when I was the one paying for it.
I can pinpoint the exact moment it happened. I got a phonecall, I was asked to come into an office because someone had to talk to me right away. Then I got some bad news.
I was told I’d lost a loved one to a land mine.
It was the end of so-called “young-adulthood”, when you feel you still can change the world, when you still feel invincible (to some degree)… when you still have the guts, the nerves, the drive to take on the world by the balls.
I became far more of a cynic that day – and lord knows I was one to begin with. Lots of little things, before, had started to mark the turn to adulthood: independence, purchases, but nothing had triggered a sudden, total mental shift. This event, however, did. Sadly, the years that followed sucked even more ass.
Thankfully, things have improved since then 
I wish I had my happy Orthodox man smiley.
When I was in high school, and some of the boys wanted to be called “men,” it just struck me funny. A 17-year-old man? I didn’t really feel like a “man.” What made me feel like an adult was when my father’s friends started telling me to call them by their first names. When you’ve been calling someone “Mr. P-----” for years and he suddenly says, “Call me Ed,” it’s very shocking.
I still felt like just a larger kid for quite a few years after that. Holding my own child (and three months ago, holding my first grandchild) certainly made me feel more adult. As many other posters have said, though, the biggie was after my parents died. I was having a plumbing problem in the house, and I’d actually picked up the phone to call my father for advice when I remembered that he was dead. Damn. It’s been five years and I’m tearing up just typing this. Yeah, it’s my responsibility now. I’m a grownup.
Actually switching from the classic rock stations to public radio can be hipper, musicwise. At least it’s better than hearing “Double Vision” and “I Can’t Get Enough Of Your Love” for the millionth time.
Making a very painful decision to do the right thing (not the easy thing or the comfortable thing) and then seeing it through to the end because it was my responsibility and nobody else’s.
My parents were out of the country for several months and I was taking care of their house and their lovable-but-old dog. I had to have the dog put down, so made the arrangements, took her to the vet and held her while they put her to sleep. I felt very much like an adult that day.
When I got married (21 yrs old) and in my mind was responsible to/for another human. I had been financially independent for a while, but only for myself.
Feeling like an adult defiinitely is a series of increments (I’m 41 now). Some of my more noticeable milestones were my first apartment at 18 years old, paying off all my student loan debts at about 31, buying my first house by myself at 32, getting married at 35, losing my last grandparent, and the death of my dad. The death of my dad was particularly interesting; there was any number of problem aspects to it that we had to work out as a family, and everything that I said was going to happen played out exactly as I had said. I don’t know if anyone else in my family noticed, but I sure as hell did (I’m not the oldest, and my opinions were pretty much disregarded).
I’ve also noticed, working with a bunch of kids at this last job, how much better at life I am than they are now. That might sound a little arrogant, but it’s absolutely true. I know where my towel is now.
Dad Died…then Grandpa died…then I realized I was the oldest male member on the family tree. 
Everyone in my main friend group has been friends since we were teenagers. We’ve all got good jobs and fully adult lives, but it’s still hard to see each other as “grown ups”. Together, we’re still a bunch of silly teenagers.
Last year, three of the 7 original friends got married, and I’ll be getting married in the spring. One friend got married in June. The day after the wedding, he and his new wife gave us a tour of the house they’d just bought. While we were standing around in the kitchen, his Dad calls, and they proceed to have a frustrating conversation about his father trying to find the street.
Friend hangs up, sighs, and says, “It wouldn’t be nearly as difficult if he’d just wear hearing aide.” I thought about my parents always complaining about my grandmother taking her hearing aide out. Behind me the new wife and another friend are talking about property taxes. And I thought, holy shit, we really are adults.
(I had another friend say to me that she realized she was a grownup when another friend said to her, “I’m pregnant,” and she realized for the first time that the proper reaction was “Congratulations!” and not “Oh, honey, what are you going to do?”)
I think that being an honest-to-goodness grown up hit me when guys that I used to date started dying from something besides traffic accidents.
That and the white eyebrow hairs that I have begun to pluck.