note There are some stories with shota's that are older than 18. Selim Bradley, Mitsukuni "Honey" Haninozuka (after he turns 18). There's also immortal shotas that are older than most.
I was thinking on other terms, like "Fake Shota" or "Shotajiji" which work similarly. Though I think "Shotajiji" could be a tag on it's own. I've seen "Shotajiji" used in the context of supernatural beings that are ancient and shotas. Like Selim Bradley/Pride, or fox spirits that take form of shotas.
"Fake Shota" because if a shota is considered to only apply if they are a certain age, then they look shota but are not, hence "Fake Shota"
But I think "Legal Shota" is simple enough to understand and it covers the (not many) mangas I've found on the site that have a "Legal Shota".