From what I've witnessed among people my age 16/17 they've only started putting on weight in the past couple of years and lot of it is down to laziness and greed.
Both boys and girls have been putting on weight (But to be honest it's more noticeably girls as far as I can tell) and it seems to directly correspond with them giving up the sports they used to do as a younger kid.
I mean seriously. All I have to do is go on my twitter and half the girls on there are constantly moaning about not having a McDonalds or something.
One thing I would say though, While in general I notice it's girls more than boys putting weight on at this age, the boys that do seem to be, are doing so in great quantity. Like serious big changes. While the girls seem to be more gradual and not as extreme.
Correct me if you think I'm wrong but this is just what I've noticed.