Ok, heres the deal.
ACA is the main org for Colorado, there are few USAC events here, mostly collegiate events that have a few USAC Categories. I think the City Park race was a collegiate event that had three USAC men's categories (P/1/2, 3s, and 4/5s).
It costs $10 for a one day, $45 for a year license.
In ACA beginner men start out at Cat4, or Cat4 35+, there is no Cat 5
There are a few races in March but it really starts up in April.
A great place to learn is the training crits on Tuesday nights in Golden. You don't get pulled if you get dropped and you can catch back on when you get lapped, don't ask how I know this. I belive these start up in May.
A few road races have newbie categories I think one at the end of April and another at the end of May. I am very interested in these, BF team??
There are a few races mid summer that have citizen categories that are another nice way to try it out. But they are late July/early August.
There are weekly TT series at Cherry Creek in April May already sold out and another in Boulder/Lyons in May June.
I have done some of the practice crits, some of the TTs and a few citizen events the past few years. Even tried a few of the real crits/circuit races on the weekends.
PM me if you like or share the wealth and ask away here,