Here's the now deleted wiki for the definition.
It appears this tag has been used in the past, but at some point it was nuked. I've thought about it and tried to search for it and I don't think this can be substituted by any other tag easily without being a pain in the ass. Of course the explicit posts would only show up to users with gold+ so in theory tag limit is not an issue, but that doesn't account for safe posts, and even for someone with no tag search limit it's a pain to find all the results.
age_difference milf has all kind of unrelated stuff like bestiality, futanari and yuri
age_difference child and age_difference shota are mutually exclusive searches, and an OR search doesn't work because "child" is not gendered. Even here there's several hundred yaoi/yuri/futa results, like for the previous result, which is pretty bad if you're specifically searching for a hetero fetish
age_difference hetero doesn't really work because loli posts make up like 1/3rd of the posts in there. So you'd have to do age_difference hetero -loli, which again doesn't exclude safe female child on male adult posts.
Lastly, age difference is not just child on adult,it could apply to any other random thing like old on adult and other pairs, so it's not a good way to filter for this kind of specific result.
If there's no argument against the tag I'll populate it.