Dude, it's perfectly possible.
I first fell in love in 3rd grade. Yes, believe it, I was so in love with this kid from my class until 7th grade. And to be honest in that period of time I even got to cry for him. I know when I like someone, and when I love someone. So pretty much I had actually fallen in love with him.
And they need to understand, you just don't "choose" when you fall in love. It just happens, any age really, when the feeling turns out to be pretty strong, and you sense it.
So yeah, whoever doesn't really believe you needs to see that if he/she hasn't fallen in love at young age it doesn't mean not a single person in this world hasn't either.