Is age a big thing in a relationship ?

i need some advice , i ask my friends but they wont tell me their opinions straight because i think their worried about hurting me. there is this guy and ive known him for a while now ,we both really like each other.. ( well were crazy about each other lol ) but there is a three year age gap. i no it doesn't seem like much of a problem but im in 15 and he's 18. most of my friends think that we should just start seeing another , age is just a number, but some f them completely disapprove. i no my parents would hate it and i don't think his parents would take it to well either.we've tried not talking but its horrible. is age just a number . ?