Here in California there aren't any weight classes. It's like this...

Professionals
Class A amateur...the good guys
Class B amateur...the intermediates
Class C amateur...the novices...I threw in Class C for most of this season, I was the old guy!

Masters 40 - 49 (years, not I.Q.)
Master 50+

Sometimes the Masters get divided up like this: 40-45, 46-49, 50-60, 60+

It kind of depends on how many guys show up. Note that here in Nor Cal we have a solid contingent of women that show up to throw. At Woodland I think there were 16 ladies on the field...all in one class.