He's older than me

He's 18 I'm 15, everybody is against us being together but it just makes us stronger. Is it wrong to be in a relationship with an age gap because to starting to effect me. I hate people bitching about us behind our backs. We're happy, why can't people accept that. The other problem is my mum, we have hidden our relationship from her for the past year but she is starting to suspect and she is the kind of person who would disapprove. I don't know how to make people see how happy we are and how to stop people from being judgemental and nosy. Any feedback would help thanks x