Most teenagers that I meet in physical betting places share the same story. They said sports games exposed them to gambling. I guess that they were exposed to gambling ads in these video games, or they were betting among themselves. It is common to see even children who are less than twelve years old placing bets on who would win a video game match or fight.
In teenagers, I think playmates have a role in how they can get to know about gambling. Gambling advertisements might be another factor, but I believe the influence of friends is strong enough to invite others to try betting.
My initial gambling experience was also related to curiosity and friends' invitations, starting with betting with friends, but after that, I created my own account and enjoyed gambling on my own.