Heck, 39 isn't old!! In the regional series, I won the Open Expert Class and 4th overall the year I turned 40. This year I turned 45, and am switching to the WORCS series, region 1. Probably start in the Intermediate classes for the first few races, but will no doubt find my way back in the expert ranks. Just can't stand to be a sandbagger, and hate the age classes.
By the way, I've ridden both 250's and 500's in the woods, and prefer the KX500 with a flywheet weight (sorry Paul) geared 14/47, as it's a tractor that never quits.
Rick