"old people are less attractive than younger people"
This is not a fact, it's an opinion. Qualifying statements like this with, "it's widely held that" or "the current beauty standard is" etc. will make clear that this is an opinion, not a fact.
Attractiveness is subjective. By making subjective statements but phrasing them as though they were objective, you're being misleading, and there's no need to throw insults at someone just because they have a different subjective opinion than you do.
Many people think both gray hair and age are attractive. In fact, it's one of the most prevalent aspects of feminist discourse that a severely disadvantaging aspect of the male gaze is that women are considered by patriarchal norms to be less attractive as they age, while men are considered increasingly attractive as they age. This has changed somewhat in recent years, but only in that women are now increasingly still considered attractive as they age as well, so that negates the idea of "old people are less attractive than younger people" further. There are terms like "silver fox" and "milf" and "dilf" etc. that, while problematic in some ways (and discussing that would detract from this post), nevertheless indicate a value on attractiveness based in aging by virtue of this language being necessary.
I don't think grey hairs affect attractiveness very much to most people. I also don't think attractiveness is the only thing worth considering here. For example, grey hairs can have a connotation of comfort, even if they aren't sexualized. But I don't think the point here is how we see them in others, but how we see them on ourselves. Aging can be scary when it happens to your own body. Your joints will start to hurt and be less resilient, your energy wanes, and your body slowly starts to fall apart. As a child you have very little frame of reference for life and experiences, because everything is new, but as you grow older, you start having a past and that's all you have to compare an unknown future to. It's uncertain and scary to know that you used to be able to run fast and climb things and now it's harder to do that, you have to stretch more and can still do less, because your body is changing and mortality is rearing its head. Grey hairs are often a sign of aging, and the reason some people don't like grey hairs is because it's a reminder of their mortality and the phyiscal decline that aging comes with.
