Vet is over 30, any size bike. Opens start the best. 250's usually competitive. 125's the most fun, because you start bad, but ride the quickest lap times. I mostly rode 250's, made it thru VetIntermediate to Expert my first 10 races, but didn't make regular 125 Expert until after 300 races. Lucky for me, it took only about 40 Open races to make Expert, and after that, nobody enforced it for different sized bikes, so I just transferred to 125 and 250.
Vet is safe racing, OldTimers safer, at 55.