Well Paul, most 20 year old women have started having periods. They have them every month, not just once. Sometimes girls start having periods as young as 10 years old. That doesn't mean they are done with puberty at 10 years old, but they are in it. By 15, 16, a girl is out of puberty. Her body will still change and grow, just like a boy's would.

A 20 year old woman with boobs has had her period, probably for a number of years.

the average age a girl gets her period is 14

so has a 20 year old woman had a period?

yes