Is it normal to have (mutual) feelings for a much younger boy?

My friend recently told me he's been in love with me for over a year. He's 7 years younger than me (I'm 25, he's 18, so the gap is a bigger difference than it would be if he were a little older) and I feel very weird about it, but I've given it some thought and realized that I'm also attracted to him, as well as caring about him deeply. At the same time, I feel guilty for reciprocating the feelings, because he's so much younger and I've been kind of an older sister figure to him.

He insists that it's not a big deal, that the number of years doesn't matter as much as how much we have in common, that he's mature enough to know what he wants, and that the fact that he's a legal adult negates any issues with the age gap (I disagree). I honestly don't know how to feel about this situation or my own feelings, because despite his age, there's a lot of mutual respect between us and on many levels I see him as an equal. But at the same time, looking at it from the outside, it seems pretty inappropriate.

So, what do you think:

Is it normal to be attracted to someone despite that big of an age gap? And is his viewpoint, that regardless of age it's okay for two adults who are attracted to each other to have a relationship, normal or do you think that's just him justifying that he wants to be with me?