Who says teenagers are underage? The age of consent in the U.S. is a state issue. And across America that age ranges from 14-21. (And in Colorado, it can go as low as 12, as long as the person can get parental approval for marriage) Depending on where someone lives, they might have been talking about completely consensual and LEGAL experiences.
Furthermore, why is the world obligated to make rules simply to make you feel safe and happy? If you don't like a conversation, LEAVE the conversation. I hate to break the bad news to you, but you are NOT the center of the Universe.
If you need a safe space, your mommy's basement is where you should hang out. But if you are going to hang out in the real world, even digitally, you should get some thicker skin, educate yourself on the actual laws of the land, and expect to hear sounds (or read words) that don't conform to your own world views.