tl;dr - the first two paragraphs are the important part, the rest is just personal.
There's no way of telling for certain how another person feels (in that sense you can never know for sure that your partner feels anything at all), so there's nothing to accurately compare love to except to your own past experiences. You don't know if you're more "in love" than another person, or if that three week relationship between those thirteen-year-olds really is shallow, because all you have to compare to are your own relationships and how strongly you've felt towards people in the past.
Older people have generally been in more relationships than younger people, and it's because of this experience that some despise younger teenagers saying they're in love.
Personally, when I was thirteen or fourteen, I used to be irritated that older people always assumed I had no capacity to think deeply. I'm sure other people have had the same thoughts. It wasn't until I was about fifteen or sixteen that I could look back and see how naive I actually was, as much as I wanted to deny it. I also came to the realisation that I'm very much still naive now, in ways I don't know and won't know until I'm older. Maybe it's because of this that love felt more real at sixteen than it did at fourteen.
There's no clear line that you can step over and know that you're in love. Some people refuse that they're in love despite strong feelings towards people because they're saving the word for "the one". I did that at a younger age, and then figured, "what if the one never comes?". After a relationship where I considered myself in love and then after it realised that my ex wasn't the person who I thought she was (we met on a camp which two schools went on together, so we were finding out about each other in the months following the camp), I wanted to take back the word. "That wasn't love," I'd plead to myself, but it was. Maybe I was in love with a girl who wasn't actually there at all, but I said I was in love and I couldn't take that back just because I'd made a mistake. So since then I've just called everything love, I don't see any point in trying to define it because there is no way of defining love.
The closest definition of love I have found was by another user on this forum in a thread around a year ago:
Real love is bittersweet: The joy of being around them and the lack when they're gone.
Hopeless to understand and simple to feel. There is nothing more beautiful.
Hopeless to understand.. it's not really a definition at all. But it's still very accurate.
