I am almost 44, but don’t really feel grown up. But I will never forget my first truly grown up act- and it’s not college, marriage, kids, mortgage, getting sober, or the usual stuff. In the space of 4 days, I had to put both my dog and my MIL’s dog to sleep. It was awful, but an adult decision had to be made and I had to do it. It sucked rocks.
That was the day I became a grown up. :mad: Still wish it hadn’t been necessary.
Edit- I was in my 40’s.
Older than 27 I guess.
I’m 34 and don’t feel like a grown up, although I appear to be one at first glance. I own a house and two cars and have lived alone since 18. I’m a single dad to a 6 year old girl so I’m dad, mom, cook, maid, taxi driver etc. I never get “me time” so all the things I want to do are on hiatus for a few years, or possibly forever. I don’t really do anything “ungrownup” and I’ve done many of the things that others say made them feel grown up.
In theory and on paper, I’m a grown up, I just haven’t realized it yet.
I’m 59 and still waiting.
If growing up means I must be anything that I don’t want be, I won’t grow up, never grow up, never grow up, not me!
I answered 23. I thought I was grown up.
I was wrong though. 
I’m 42 and I still feel like I’m faking it.
14
Long story about my father and how he was an adult at 14 and I was convinced I had to match that growing up.
To this day my mom asks
What are you going to be IF! you grow up.
Nope, aint gonna do it
By the time I hit 30, my kid/student lifestyle was coming to the end.
That’s how it was for me too. My dad had mild dementia and a slew of other medical problems when his wife left him. So 21 year old Fuzzy Dunlop started doing his best to take care of him, starting with forcing him into a psychiatric ward because he wanted to kill himself. I still have his suicide note. 
It’s a weird lonely feeling but I definitely knew instantly that there was no going back and no grown up who could take over when things got tough.
It sort of amazes me on some level that there are people in their 40s and 50s with healthy parents. I get that it’s actually normal on an intellectual level but I really can’t comprehend what it would be like to live that long without that responsibility.
When I turned 30, it felt like I had some real substance. No more darling of the office or whatever, but a professional with… gravitas? I started acting that way then. More reserve, more confidence, more repose.
On the other hand, I’ve always been a little butch so now I’m a 50yo who dresses like a teenage boy. 
I’m 23 and still feel like a kid even though I moved out at 18. I’ve been done with college since 2010 and have been working full time since then. I guess I do adult things like cook, pay bills, go to bed at a reasonable hour, but I still don’t feel it.
As I say to my daughter, someone in this family has to grow up, and it’s not going to be me. 
I’m forty-nine-and-a-half.
I had a councellor tell me I have been “coping on my own” since the age of seven - so maybe that counts for something.