Hey. i think a good thing to do would be to talk to one parent first, individually. maybe your mum? she might understand a boyfriend problem better. But it doesnt have to be a bit deal, you could just drop it in.
or you could build up to it slowly.
you could start talking about this guy, and drop his name into conversation, and if anyone mentions it, you can tell them. make it fun, a joke out of it. but, always be honest about his age.
if you think your parents might overreact about his age then i think you should definately talk to them seperately, and tell them how well you know him, and how hes always treated you well and with respect.