Note: I am prepared to be a hypocrite, saying the show is not for me when I'm twelve and it still being one of my favorites.
Fringe opens almost every week with a gruesome horror-ish twist. The twist can continue into the rest of the episode. Also, really nobody is a great role model for your kids. Peter is... well, he's Peter and he's too mature for kids, Walter is insane, and Olivia Dunham is ruthless.
Walter normally solves the episodes with "fringe science", which is NOT to be confused with hard or even realistic science. It can also be gruesome. It will not make much sense to younger viewers. Walter is the main sex reference maker, although being insane I guess you can let that slide. There's no nudity, although there are unclear sex scenes. So yes, people do get laid.
I would choose to shy away from this if you have kids, and take a moment to honestly evaluate how much your teenager can handle. If they can handly gore and adult concepts, then Fringe is one of the best shows out there in my opinion. If they can't, then you might want to look somewhere else.