So I'm not sure what your question is - how old was everyone when they first had sex?
It doesn't really matter when they did. What matters is that 20 isn't that unusual, and that you did it when you felt comfortable, and when you were ready.
Also, I wouldn't necessarily believe all your friends, especially the one who tells the story about his sister. That seems far fetched, and also illegal. It doesn't make him sound cool or anything.
Lots of people - including me - had sex before they were really ready. It's not that great. I'd have much rather waited a couple of years. I was totally unprepared.
In all areas of life - sex, relationships, jobs, finances, etc. - people are on different paths. None are wrong. It's just different. You could be 30 and a virgin, and it doesn't make you a loser.