Define "teen".
Also, thanks to puberty every teenager has a different level of maturity. And that's what it comes down to.
Would you believe a 10 year-old if they said they were in love? Of course not. Why not? Because they're not mature enough to be able to recognise it and to view it as objectively as possible.
In reality, it depends upon the individual. The vast majority of twelve year-olds are far too immature to possibly be in love. By the time they reach nineteen, they will be mature enough. Certain people buck this trend.
In my opinion, "teen" love would be around 13 or 14 (well logically speaking, thirTEEN, fourTEEN and etc). At such a young age, most teenagers would still have no idea about how the world works. In addition, many teenagers are now influenced by many different social factors such as peer pressure, teen actors/actresses (Zac Efron: HSM star ring a bell?), television series, and even more simply "teh internets"... and the list goes on. Basically as many people above have posted, young people's minds are always moulded and shaped not only by family but also external factors which no one really can control. So for example it may seem cool to have a girlfriend/boyfriend at a young age such as 15 because you may get a higher social status in high school, and this may have been influenced by a television series such as the OC (I'm being very general here).
After seeing quite a number of failed teen relationships from my school, I can honestly say that they may be "fun" and "cool" to have during high school, but in the end the effects can be harmful not only psychologically but academically. Wouldn't it all seem a bit pointless to be in a relationship for 3-4 years and then a sudden (and usually quite painful) break-up right before one's HSC can cause mental and physical stress. It'd affect your HSC performance standards, which would not only hamper (or destroy) your chances at university but also in some extreme circumstances if either party cannot handle such a break-up they may cause themselves self-harm.
In simple terms my view is this:
-Teen relationships never work out, even if they do in the end most likely you'll end up as "just friends". Wait till university, the world is a bigger place than you think.