Because I was in a serious long-term relationship with a guy who was 15 years my senior. When you date someone who is that much older than you you start to realize that health problems you don't even dream about you have to deal with because of your partner, and especially in a long-term relationship that gets very stressful. He for instance had been a smoker for something like twenty years and was overweight - not obese but overweight. These were issues that were catching up with him over the time period we were dating.

You also start to think about stuff like kids. When you have one partner that's a lot older, suddenly you are faced with the fact that your kids are going to have a much older parent. Depending on the gender of the parent that can limit the window in which you can have children and force you to rush into a decision. (That wasn't my case, thankfully, but with an older woman it would be.) Even when actual fertility isn't on the table, then think about what it's like to have an older parent: yes, older parents can be successful but they can also be out of touch and parenting can be harder for them due to their age. Really depends on physical condition. So an older partner can still cause the couple to feel like they should have kids sooner rather than later even if the partner is a male.

Those are the two things that spring immediately to mind when thinking about it in general and also considering about how my partner's age affected me. In addition I was quite young and he was, well, middle-aged. When you have someone in their mid-thirties who's happy dating a 20-year old (and we'll gloss over the other problems I had at the time) I think at some point you need to start wondering about the maturity level of both parties involved. I couldn't even drink. I hadn't finished college. I'd never had a real full-time job. I was someone who hadn't experienced a LOT of what life had to offer. I think it's safe to say most people in their mid-thirties would have found me immature, relatively speaking.

I think that difference in maturity levels gets smaller as the age of the younger person in the relationship increases and should be considered less of a factor if at all.

In my late teens and early twenties I found myself dating a lot of VERY older guys, 7 years older or more usually. After I got out of that LTR w/the guy 15 years older than me I decided I was going to try dating closer to my age and see how it went, that it might be a good guideline for me to follow. I guess it was an experiment in being normative. I still tend to go for slightly older guys, but it's not, like, significantly older guys with low maturity levels (which is the demographic I was hitting before).

So that's why it's personally important to me and also why it might be generally important to people. Also with age I think there comes a push for a committed relationship/marriage that younger folk may not want to consider (Hi! That'd be me!)