The biggest asset you will have in learning motocross is having a very tight and fit core.  Your abs, back, etc.  This is where most of the work will happen.  If you go to youtube, there are many videos on learning motocross.  There are tracks all around the country that offer training, practice runs, etc.  Most of the motocross events out there are amateur.  Start with a smaller bike and build up from there.  Build up your core and hit the track.  Take a lesson.  As long as you're fit, I'd say you can at least practice motocross (racing) well into your 40s and 50s.  The big air tricks however.... there is def. an age issue.  30s max... unless you're a pro who's been doing it for 20+ years. 

You can also go to xtrm.com.  Is a social network for action sport athletes and fans.  You can join and poke around the system... ask the motocross guys themselves what they think...