Would you be grossed out by this?

If he's trying to realistically date an 18-year-old, then its likely that he is trying for a purely sexual relationship. He should know at his age that the only thing you two could possibly have in common is that his penis will fit in your vagina. I can, however, understand that the companionship provided by younger people is special because they are exuberant and fresh, which is a delight to us older guys. On top of that, a woman is at her physical best at that age and having sex with one would fulfill every middle-aged man's fantasy. I work in retail around a bunch of cute 20-30 year old women--I'm 47. I love being around them and even flirting with them but I know that I could never date any of them or even just try for sex with one, as great as such an experience might sound. He needs to check himself. Anyway, you don't owe him any explanations. Just kindly say "No, but thank you for asking." If he persists, say the same thing each time until he gives up. If he pressures you for a reason, you can, as others suggested, lie and say you have a boyfriend, or you can just tell him the truth and say you'd prefer dating someone closer to your own age. PS--He may be nervous because he's an immature, overweight, lonely 40-year-old with dreams of re-invigorating his youth by dating or having sex with an 18-year-old. You should graciously decline.