No need for a long preamble.
I am am soon to be 40 and would love to hear the wisdom of older Dopers.
Turning 40 is better than the alternative.
For me at least, it seemed to be a somewhat magical age where I really no longer care at all if I’m not up-to-the-minute on the latest trends or if some teenager somewhere thinks I’m uncool. Of course, I never really cared too much about that stuff to begin with, but whatever shreds that were left have completely disappeared.
In other words, you get to completely embrace the you who you are. 
Happy upcoming birthday!
I’m 45…trying to remember turning 40. Well, there’s a woman with whom I work…she’s about 8 years younger than me and always trying to give me shit about the fact that I’m older than her. She orchestrated some ‘old folks’ jokes/pranks at my expense etc. when the magic day came.
After the fact I said, “At 30, you realize that those big dreams you had aren’t going to happen. At 40, you’ve long since accepted that.” I didn’t mention that it frees you, in a way, to become the best at what you (already) are.
Not long ago she came up to me and said she’d turned 37. She said, “At least I’m not 45.” I countered, “OTOH, as a man I grow more ‘distinguished’ by the day. You just get older.” When will she learn not to mess with me about this?
Groucho said you’re as old as the person you’re feeling.
OK, seriously: IMO it’s freeing.
You’re within a year or two of realizing that it’s easier to focus on what you’re reading if you move it away from you a skosh. This is the first sign that you’re about to start needing reading glasses. Buy the drugstore kind in multiples.
It gets worse.
And I stared liking the Blues.
Started liking the Blues at 40, dammit.
The hell was I talking about?
Take care of your prostate. If you don’t have a prostate, take care of someone else’s.
My eyes started needing bi-focals the day I turned 40, I swear. Get used to your body just falling apart. Other than that, my forties have been great. 
Us runners love turning 40 (or 45 or 50, etc).
Moves us into a new age division.
Two days after turning 40, I won my age group in a local 5K. (No applause, it was a small race.)
So, you were planning to be here 16 years when you picked your user name?
I am actually 37.
I am just thinking about were I am at. My life seems to be at a crossroads. Business, friends, family, marriage. I feel a new identity coming on.
I remember a thread about a similar age group and can not remember enough about it to search for it. I think it was advice for thirtysomething thread. I loved it and thought the responses were great.
As for my user name, 56 is my favorite number and has been for over 20 years. I have considered changing it to -23 or a fraction for about a year or so.
My advice: if you have any extra weight on you that you aren’t happy about, you need to put a plan in motion to take it off pronto. Weight becomes very difficult to lose once you get into your forties, at least that’s been my experience (female by the way). I think it’s the hormonal changes that are gearing up to take place that are partially to blame.
Overall though, this phase of life hasn’t been bad. Fifty? That’s the scary one IMO.
No lie about that weight thing! It gets harder and harder to keep the pounds from edging ever upward! It is such hard work now!
Just turned forty, and as Featherlou said the eyes went wonky pretty much to the day. As for the rest of it, I’ve got a 16 month old to keep me young and another on the way to remind how old I’m going to be when I finally don’t have to pay for anyone’s tuition. Based on that, I’m pretty sure the next twenty years or so will involve alot of wonderful, and crappy, new experiences. Enjoy them, there all we’ve got.
Yeah, it’s funny what you start developing an appreciation for in middle age. Often one begins to develop an appreciation for the finer things in life.
With me it was (note the was 
) the blues, smooth jazz, flowers, art, really good quality antiques (and workmanship), good food & wine, bourbon and single-malt scotch, and more than ever, beautiful women.
I’m 60 and cared nothing for any of these things at the time I turned 40 (well, women would be an exception, but not necessarily the really beautiful ones).
I’m turning 50 next year and 40 didn’t bother me on an emotional level but things do take longer to heal now than they did when I was younger. I used to be able to party like crazy for days and get up to go to work the next morning but now one night of that would wipe me out for a week. You start to figure out that you aren’t indestructible and need to take care of your body.
It did motivate me to make a fair amount of changes in my life because at 40 there is no denying that you’ve reached middle age. I had gotten out of tech to start up a bunch of homeless services when I was 30, and at 40 I realized that I was burned out, had no savings anymore, crappy minimal insurance and nothing planned for retirement. I turned it all over to someone else and went back into tech and started stashing away money to buy a house and build up retirement savings. I also splurged on a long trip to Europe because I figured it might be the last chance that I’d have while I was still young enough to do strenuous things. I lost a huge amount of stress-weight and now I’m probably healthier than I’ve been in years and years. Weight does take longer to come off too.
Someone once said to me that career-wise, in her 20s she did what she was supposed to, in her 30s she did what she really wanted to, and in her 40s she did what she had to to retire eventually above the poverty level. I think that’s close to what I did. I just had the opportunity to retire early at 49 and took it. I can’t begin to tell you how great it is to have years stretching out ahead of me with no need to work. We aren’t rich but we’re doing fine and all of that saving and boring tech management work was well worth it. I’m not at all worried about turning 50 and I don’t feel old at all. Every stage of life has it’s benefits and drawbacks. I think.