Stretch and exercise more, my dude. I'm on the other side of 40 and still feel great - none of the aches and pains a lot of my colleagues talk about (though I'm sure those will come - no one gets to stay young forever). But I'm also a lifelong martial artist and I work out five times a week, each of which involves a solid 20-30 minutes of stretches.
I had a sensei who retired at 95 - not from training, but just from running the global organization, because he felt he was getting a little old for it. He was certainly no spring chicken, but if I'm half as spry as he was at 95 I'll be a happy man. I have another sensei (different art) who, at 76, can knock your head clean off your shoulders if you put a staff in his hands. I have yet another sensei (of yet another art) who is 67 and can still clean my clock in sparring.
If you take care of your body, it will pay dividends, even into your twilight years.
Kill 1 man: You are a murderer. Kill 10 men: You are a monster.
Kill 100 men: You are a hero. Kill 10,000 men, you are a conqueror!