Anyone can love, but the type of love (if you will) varies for different people and ages - for example a 5 year old loves his parents, but in a different way to a couple who have been married 40 years.
You can know what love is at any age, but your idea of love will change with age. Like what others have already said, a highschool couple can be in love and grow up and get married, but the way they love each other will change a lot with time.
It's unfair for an adult to say teenagers can't be in love, in fact, it's unfair for anyone to tell someone else if they're in love or not. You weren't in love or mature enough to know what love was at 17, doesn't mean it can't happen to others. It comes down to the old 'everyone is different' thing.
I don't think anyone can really define what love is until they've experienced it. That doesn't mean you have to get your heart broken to know love, but you know what it is when you've experienced it. I love my family and my friends, but I haven't been 'in love' with anyone. That's not saying I'm too young to be in love, I just know that I haven't been in love, it's one of those things that you just know when it happens to you.