How do you know when your in love?

Me and my boyfriend have been together on and off for a year now but it was officaly 5 mounths yesterday also his birthday. we say I love you to eachother all the time but im just not sure if its real. Also should I be freeked out when he says I want to...

be with you forever? Grow old together?...sometimes I feel too young too because im only 17 I don't know what too think...?!?

Sit down and talk with him about it.
If you want to last, and not break up, you would want to tell him how you really feel instead of always keeping it to yourself.
Voice your opinion, that's what relationships are about. To actually tell the person everything, and them to accept it.