i'd be a bit suspisious of his intentions and whether or not i was some kind of "novelty"/trophy girlfriend to him.
but if you are having doubts about whether or not this guy is too old for you, then thats a problem right there. if you really liked him i believe that would over-ride this age issue.
i have doubts about a relationship in which one of the partner is a teenager and the other is significantly older. you are both at very different stages in your lives. a 17 year old may still be with his/her parents while the 28 year old is a fully fledged individual, living on their own with their own career and financial responsiblities. they are immersed in the adult world and have adult friends. on the flipside, the 17 year old might not have even completed their education yet and are still living off their parents. technically, they cannot go out to places like bars and clubs with the older partner.
i can only forsee many problems coming up and frustration on the older persons side at their younger partner not being able to really join into this adult world (e.g- the clubs, understanding what it really is to be an adult etc). this may not be the case for everyone but in my experience, a lot of these kinds of relationships fail that way.