I would suggest you start at the lowest category you can find locally and race to get upgraded. This might take you quit a while (or not, depending on your innate ability and how 'smart' you train, adjusting your approach to your age). You will probably then find age categories are regional, state and national championships, etc.
I'm not from the US but specialize in coaching older riders and write on aspecgs of it etc. It is my opinion that the standard is relatively higher as you get older - i.e. a much bigger proportion of the field is in top shape (relative to their age of course - the higher you go in the age groups, the more 'hard cases' you find in the bunches).