I get my ass handed to me regularly by 55-59 year olds. Haven't seen a 60 year old whoop me yet. But I don't ask everyone their age and I haven't ridden with everyone. Still, I would put the magic "going downhill" number at 60. Saying that I have ridden with several 70-76 year olds who do a decent job holding their own.

I think the comment about 18 being peak might be in reference to the body's ability to improve and repair itself, producing the quickest results.