age gaps can always be a funny thing.
i found that with my best friend she seems to date older guys.. so at the moment shes 18 and hes 23 (im pretty sure thats his age) but he doesnt act like an adult. hes still a little shit in my opinion.
so it all depends on the person. someone can be very mature in their 20's. others can still act like their 16/17/18. so it all depends on his personality.
if hes more mature and serious about things... yeah things could go wrong. because your still young and havnt experienced life as much as he has, so u might find that you have communication barriers in some things. but as u said he wants to wait till your older, theres is a chance that in that time frame he might lose his feelings for you. (im just saying its a possibility). in a few years time, he's going to want to settle down, if he asks you to settle with him, are you?
why i ask this, because ive seen first hand what this age gap can do to someone. my best friend was the life of the party. she was the biggest flirt you'd ever meet. she always planned to go out with me and we'd live a single life together (this was when we were back in year 10, bout the same age as you) and yeah. she met this guy, who was older. and he kinda took a great influence over her. he influenced her not to go out, not to party, not to flirt with any guys, even a trip to the internet cafe cause dilemas because he was paranoid she'd flirt with another guy. so now she has reserved herself ALOT and now shes moving into an apartment of her own, and prob her bf will follow. she had no single life. no time to go and explore life's posibilities.
in my opinion, its a bit sad. it just places barriers.
you just have to make sure that your careful and not be pressured into anything. you are still young. take this relationship lightly at first. and hiding something like this from your mum is very risky. if your relationship becomes anymore serious, u must tell your mum. if you dont, you will have a very hard time trying to get her to forgive you in the long run.