Harry Potter's aging process

Am I the only one not seeing a problem here?

I think it's called puberty. It can be pretty sudden

It's also an issue of just not noticing it (in real life, course the movies are different). You see a person every day, so even though they are getting taller and whatnot, its less noticeable. Then they are gone for three months and all those changes stack up and its much more noticeable that they've grown.

Could also be a time difference between the school and the real world.