Why is it kind of a taboo in for young people to have friends a few years younger than themselves?

I remember a few months back listening to two girls talking about someone they knew. this girl was 17 and frequently hung out with girls 13 and 14 years old. The girls talking about it thought it was weird and she should hang out with people her own age. I was wondering what the big deal was. I mean, who cares?

It didn’t take long, though, for me to realise I frequently look down to girls who have friends more than two years younger than themselves. Talking to other people, I have realised that this attitude is very common around people my age.

So why do we find this a problem at this age? is it because of big developmental differences at this stage in life? This stigma does go away doesn’t it?