If those are really the only two reasons you can think of, I suggest you’re having a failure of imagination. (For what it’s worth, I’m 36 right now and in an ideal world my dating preferences would be in the 18-24 range as well).
I’d suggest lots of people like to date very young adults because they’re, you know, still in the process of forming their fully adult personalities, career aspirations, likes and dislikes, values, etc…, and that’s an exhilarating stage of life. It can be really exciting to guide, nurture, care for, lead, and sort of take on a quasi-parental role for someone going through that process. And for that matter there are a lot of young adults (young men and women both) who want someone to fulfill that role for them, and are happy being led, nurtured, taken care of, etc… A relationship like that obviously isn’t going to be a partnership of equals, but I strongly disagree with the idea (which seems to be widely shared in America today) that healthy relationships are necessarily characterized by equality anyway. The older model of marriage (if you look at, e.g. the old Christian marriage services) was one in which the wife pledged to ‘obey’ her husband. I wouldn’t agree with the explicit gendered nature of that, certainly, but I do think that the best relationships are those where there are a clear ‘leader’ and ‘follower’. (Not that the male always has to be in the dominant role: personality traits vary a lot among men, and among women).
As for ‘just sex with pretty young things’, I certainly don’t think that’s always or usually the case. It’s certainly not the case for me. That said, I don’t think there’s anything wrong with it. Trading sex for status, economic security, etc., is a very old and bog-standard type of relationship and for that matter marriage, and I don’t see those sorts of marriages or relationships as worse than dating or marrying for any other reason. If I ever get married it would certainly be <I>largely</I> me looking for youth & beauty and I would assume, my spouse looking to be taken care of financially.
Incidentally, these types of relationships aren’t as uncommon as some people seem to think. In California as of 2004, about 0.5% of births among girls 18-19 were to fathers 40-62, about 1% to fathers in their late 30s and 4% were to fathers in their early 30s.