Hi Ms. Lane,
The answer to your question is going to depend significantly on location. From your earlier post, I guess you're in Massachusetts as is your co-worker.
As a Cat 3, 50+ Master in SoCal, I can tell you that Masters racing is generally not for beginners out here. Our 50+ races are not categorized except for district championships but a beginning racer will have a hard time with the pace and the tactics.
I generally "race" the 50+ race and then sit in the 40+ or 45+ 1/2/3 race (someone has to be pack fodder) for additional training and grins. On a typical industrial park crit, the 40+ races are 1-3 MPH faster than the 50+ races (which average 25-27 MPH). 50+ fields out here are generally 50-70 racers while 40+ fields have 50-90 racers.
I would recommend that a beginning 54 year-old racer start with some low-key Cat 5 races or a mid-week series race, just to get the feel of a bike racing.
Of course, YMMV especially in New England. Hope this helps.