スラッシュ said:
This line of reasoning is really about making tags that are intuitive when searching as well as tagging, which will avoid people getting confused or misinterpreting the tag and getting it wrong.
Oh please, the grey_hair tag has long been used to just distinguish the coloration grey regardless of age. Even after it was freed of its status as an alias to silver_hair a few months ago, it has managed to be used 728 times on characters. It's clear there are plenty of people who would use it regardless of age, and aren't bound by the idea that it has to strictly be used for old people. It's not like people are going to be wrong if they tag an old person grey_hair and old (or whatever tag for elderly) so that older people can be separated out of the other characters tagged grey_hair.
When characters can have any color of hair at any age, then what is intuitive is that grey_hair is used for characters with grey hair regardless of age. If you're searching for old people, what would be intuitive is that there is a tag to cover old people, and what is intuitive is that when you're searching for old people with grey hair you'd search old grey_hair.
Updated