My mother is 72 and still goes off cycle touring and has even been known to potter around Grizedale on her mountain bike! A guy in our club was still riding in his 70s and I know lots of men in their 60's who ride both socially and competitively. I once went riding with some friends and a guy asked if his uncle could join us - dont worry he is late 60s he said. He forgot to mention that his uncle was in training for riding the pyrenees coast-to-coast and promptly rode us all into the ground.

If you look at the vets category in both road and mtb races it seems to be one of the growing fields these days. 

As already said there are plenty of older ladies riding bikes, most of those i know are not necessarily into high level MTB competition but certainly there are those who are extremely fit and fast in their 60's who compete very highly in time trials and have won the on going BBAR national competition.

LA who is your mum, just wondered if I know her or of her from my extensive touring experience.