My uncle who is close to 60 still races bicycles. His last adventure was a bike ride from Tahoe to Vallejo, Ca. - about 260 miles. He did it just to do it. There's no age limit when it comes to sports and fitness.
I agree with others, though, as we age our recovery time increases and the body just can't seem to take as much volume of work. Doesn't mean we have to stop, though.