Everyone has a category, a gender, and an age. Based on your name I'll assume your gender to be male. Your racing age is the age you turn this calendar year. Your category will be Cat 5 and you'll be eligible to race in any Men's Category 5 race. As mentioned above, most crits have masters races where you have to be at least that old to enter, so a 50 year old can enter a 35+ race, but a 36 year old can't enter a 45+ race.

Note that many masters races are limited to certain categories too. So typically, you'll see a category listed as M35+1/2/3. That means that in order to enter you must be a man, at least 35 years old, and must have a Cat 1, 2, or 3 license. Sometimes you'll see a M35+4/5 race, which you could enter as a 50-year old Cat 5. Often though, your only option will be an open age Cat 5 race where you'll be racing against teenagers.

Very rarely you'll see an open category masters race (M35+1/2/3/4/5). I would not advise doing such a race for your first experience.