I think because you live in a country where they "separate" school phases. One school for primary, one school for elementary, one for high school.
In some countries is one school for everything, so people kinda have contact with everyone older-younger, they chat, do sports, things like that together so yeah their personality didn't change that much I'd say.
At least when I was at high school I had many friends from other classes (younger than me) who always chatted with me, wanted to play soccer together, things like that.
As example, here, weeabos from high school even chill with elementary weeabos to talk about anime, things they like, etc. So they kinda grow up with same mentality and 1-2 years difference don't change that much.