This is more of a theory I've been cooking up, more than a statement. I can already imagine it being subject to change depending on many factors.

So to concise it, older people have to make stricter life changes to achieve the same healthy results which where easier to produce when you are younger. It is a lot easier getting and maintaining fitness at 21 than it is at 30. That alone makes it so that a lot of people who were attractive and picky at a young age may grow to let themselves go.

The reality is that both men and women define if they age like wine or milk by their discipline and lifestyle. A man in his 35 can easily be considered "damaged goods", just like a woman who is 35 can still be highly desirable to men.

So now we have more people who aged at their late 20's and higher that haven't done the work necessary to be attractive and valuable, but due to the past relationships or soul searching they have done, they now have higher expectations and better analytical capabilities, so they can vet someone a lot earlier and determine if someone is worth pursuing/keeping, or if it's better to just cut that person off.

The combination of older people not "stepping up" to making themselves as desirable as possible, coupled with their new found wisdom to have higher standards expectations off the people they keep in their life leads to older people being less desirable.