Like Andymac said, I'll be racing more with guys who are my age on cruisers, and let's face it: I'll look bad enough without getting stomped by 17 year olds!
I'm looking at this as an entrance into the sport, and more from the aspect of having fun. I'm old for a rookie, and I am wanting to do this with my kids (6, 8 and 10), and as a hobby/way to keep in shape. But I am a bit competitive, and I'll be too frustrated if I'm starting out in a class where I can't be competitive. At least if I'm getting beat by other guys, we'll all be old enough to go out for beers and laugh about it afterwards!
But, if I do get where I want to be, 20" would be the next step.