I agree with Yvens and feel that young people sometimes get love confused with infatuation.
Young people can have strong feelings for others, but their lack of experience means they don't always see the difference between love and infatuation.
Coupled, as Yvens said, with the fact that young people are very often driven by hormones, a growing sex drive and an almost indefatigable need to satisfy it, the young can quickly mistake one for the other, even with the best will and intention.
Their feelings may well be genuine, but may not be love.
As for the authority to judge whether someone is too young to feel love, I don't think anyone has the right to judge one way or the other regardless of the age of either party.
In the defence of those who profess to know "what's best", they are judging from their own experience--the only way anyone can with a subject such as this--and relate the feelings of the young to the way they and others around them acted or felt at that age. As to whether this constitutes actually "knowing" what really is best for the person or people concerned, well that's another matter.