What's to old?

Okay, so there's this boy in my new school and I kinda have a HUGE crush on him but everyone says he's too old. I just want to know what is too old? We're only 4 years apart. The thing is though it's his last year and he leaves early meaning instead of leaving in July he leaves in May/June after doing his GCSE's. Me and him get along pretty well but argue about stupid things a lot. I've liked him since around 1st term September. The first time someone said he was too old was when we were messing about and his friend went to my form tutor - bearing in mind I don't like her - saying that I was being annoying, she then immediately presumed that I liked him and said he was way too old and I should like someone my age or in the year above before restricting me from talking to him. The next time around was literally yesterday. A teacher who had known him since he started said that he was too old and not as nice or amazing as I though he was. She told me that yeah it was hard not to like him because he had a charm. He has these huge light brown puppy eyes and was always smiling and laughing. The way we met was when me and my friends were going to the library but the gate was closed so his BEST friend told me that if a called him peng (hot/attractive) he'd let us go in, I pointed at my crush who we are now going to call Vet and said well he's peng but you're not. Vet's friend immediately reacted and said that (he was asian) I would obviously go for the (not being racist but this is what he said) 'white' kid. Vet started laughing and started to go on about how peng he was. I thought it was funny so I went around calling him ugly and that lasted 2 weeks until he said that fine we could be friends. Basically I want to know is WHEN IS OLDER TOO OLD?