There have been plenty of riders well beyond 17 yrs old in that class. I think the intent is more towards avoiding having experienced racers cherrypick the class.
edit: Another choice is to go just a smidge bigger in displacement and then you can have a bike that will be decent and legal for years. 400cc fours (Yamaha FZR400) and 500cc twins (Kaw EX500) are eligible for RACE Lightweight Sportsman, and SOAR Vintage as long as they are at least 20 years old (but in the case of the EX500, it's pretty likely that no one will care if you bring a newer one ... they're not appreciably different). If it is model year 1989 or before (or a later model that is identical to 1989, example EX500 through 1993) then it is also eligible for VRRA Period 4 Formula 3.
You *could* run a Ninja 250 or CBR250 in Lightweight Sportsman (and the vintage classes, in the case of the Ninja 250 as long as it is old enough), it's just that the FZR's and EX's and occasional RZ's will outgun you on straights.
I'll be running SOAR Vintage and some VRRA rounds this coming year on my FZR400 as usual. I've given up on RACE.