On the off chance you missed the previous references - You'll be cat 5, and you should worry more about racing with other cat 5s than other 50 yr olds. Some races will have a Masters 4/5 division, and that's not a bad place to be, but if you've only done the one crit, you may do better with the other cat 5s, which will typically include a few others in their 40s, 50s, or even 60s.