For example it appears on MN a 17 year old is a small, innocent child while an 18 year old is a fully mature adult and able to take on the world.
I have seen that if a 17 year old was to date a 40 year old people would say pedophilia and wrong but if a 21 year old was to do the same it's okay? And there's only really a few years of difference between these two ages.
Another example of this is that user NoLove's opinions on threads seem to be quickly ignored and she has been mocked because she is 17 (I think) however if she was 18 mnetters would say "she's an adult" and respect all of her opinions.
There just seems to be a strange attitude to age, that a couple of years make a massive difference in how situations are handled.