I'm 45, and doing pretty well for the last year cycling. I used to race, and it wasn't a big deal when I was younger to spend whole days out on the bike. I still have that mindset and it would be easy to overtrain, but here's the basic stuff I keep up:
Food. Riding a lot, its hard to eat enough so I make a point of eating more than I feel like and I always eat immediately after a ride. There's supposed to be a little window of opportunity to kick the body right into "recovery mode" instead of letting it go into "starvation mode".
Supplements. A good multi, an amino acid complex, that joint compound stuff with MSM, and a fish oil supplement. More or less sometimes - I'm prone to trying things I hear about, and then going through a bottle to see if I notice any difference.
Sleep. This should probably be number one, but the main thing I've noticed is that hitting up over 700 miles a month, I have trouble sleeping, which immediately leads to all the symptoms of overtraining. A friend who runs recently suggested melatonin supplements as a sleep aid and they work well. So I give up an hour of non-productive evening dinking around to get an extra hour of sleep, and feel better all day.
Stress. This I've been trying to minimize for the past couple of years, with mixed success. On the job, if there's work to do I have to do it, and I'm kind of on call so it often screws up any other plans. But besides that I make a settled habit of not volunteering for stuff. My wife thinks I've become anti-social, but screw it - I am seriously not looking for things to do to fill my time.
That's about it. Sometimes I get worn out and take an easy day or two, but I don't worry that I'm screwing up. My recoveries are as good as my age allows, and that seems to be pretty good so far.