How can I change people's mind on my older boyfriend?

It's a 3 year age difference. And 18 is NOT always the legal limit. Most states allow you a certain year age difference between partners as long as they're both above a certain age. The minimum I've ever seen in 3 years. And depending on your birthdays, you may be even less than that. So what you're doing is more than likely not illegal. Point out to them that it's only 3 years. It's only because he's 18 that they're acting like that.