First time book reader, long time show fan - what's with all the underage characters?

Hey, been a long time fan of Game of Thrones (greatest show ever made including its last seasons, fight me), I love everything about it.

Finally I decided to read the books as well. Boy, they're heavy tomes! Getting through the first book, I was struck by all the differences, but one thing in particular really shocked me.

Why are so many of the characters, especially those in sexual situations, underage?

I'm such a huge Emilia Clarke fan that I drove over three hours just to get a photo of hers signed (the one from her wedding night with Khal Drogo!) and I was aghast to find out Khaalesi is just a kid... Like, this is terrible to read. This is downright pornographic and it involves kids. Why didn't George R. R. Martin just write the characters to be adults like they are in the show? I feel these books really ought to be rewritten, and perhaps if Martin can't make it in time, David Benioff can finish the last book. Without the weird sexual stuff that Martin seems really into.