In reply to Martin McKenna - Rockfax:
The main explanation for the sudden drop after age 20ish is the financial/career/social aspect. If and most likely when you no longer have your parents driving you around, funding competitions and training becomes more difficult. Most people will need to start working or head to university and it suddenly gets a lot harder to compete and be competitive as an adult. Of course, at this time in life other stuff gets in the way, too - work, socialising, relationships, career, moving away from home etc. Thankfully there appears to be growing support for young climbers today as the sport becomes more mainstream.
Martin - we both know what it's like when Dad's Taxi retires, don't we?
Post edited at 20:55