A bunch of mates are late 30's and we ride a few times a week. Friday arvo ride is normally followed by beers at my joint. We mostly do XC and tech single. We love it and manage to pull another friend into it every now and then. Keeps us fit and is better for the body than the sports we used to play. Much better on the joints. Gets us out together and the wives don't mind so much as they kind of like a cyclists body.
There are about 10 of us now who had bought new MTB's in the last 2 years. As kids we had BMX's (late 70's and early 80's). Most of us were born around 1970. We mostly bought hard tails and are now converting to duellies.
One of the good things about hitting a sport like this when your older is that you can afford to buy almost whatever you like, so long as you can justify it to yourself. I find bike shops often have young dudes saying stuff like "Hmm well these ones are expensive, and may be more than you need." And you think, "Dude I just spent 40 hours a week earning a fair bit more than the average newspaper delivery kid, It's my hobby and I don't GAF, so I'll take it, and I'll also have that and that and that".
Hey I have noticed that cruising the streets from place to place that the 40+ women walking their dogs etc will often have a bit of perv at my legs. I was a bit surprised at first but now I make sure they get a good look. Now if only their daughters could show a bit more body off in return! It's funny eh, younger ones stick their tits pout as you ride by, where as if you were driving they would be head down looking away. lol
I've also met a few late-30 MTB locals in the area (really experienced/skilled riders) and they come over and get on the piss on Fridays too, we learn heaps from them.
We tend to take it much easier on the riskier stuff than than we would of 20 years ago. The frontal lobe (which helps a person to consider the outcomes of actions - prior to doing it) is not full developed int he male till the early to mid twenties, this is a contributing factor to why teen boys do some silly things. In the female it develops much earlier. Thats why girls seem a bit more sensible at those ages. It's not all about hormones and all that shit.
We also know that our bones are getting more brittle and it takes longer to heal when your older. Additionally, we have most likely had and seen a variety of juries and can remember that 'It farkin hurts!' (at least until it goes numb, we get an anesthetic, or we pass out. It helps us stay alive.