Yes, it would be unfair. Age alone does not contribute to being a good staff member. Plus you can't solicit personal information from any user under the age of 18... and people have a right not to disclose personal information. Anyway, remember that Fandom is only for people who are 16 or older (California and the European Economic minus the U.K.) or 13 or older (rest of the world plus U.K.).

The way someone acts is usually a much better indicator of their age than their actual age as a number, so besides the privacy issue of disclosing one's age like what Azure said, I'm not a big fan of basing off user rights using a sole number. It would be kind of a low move if someone wanted to apply, has the skills/requirements and mindset for it, but can't because they're one or two years off the age requirement specifically. It's totally possible that they stick around for that time, but it's also possible that they lose interest and leave the wiki altogether, although I guess in that case it's arguable if they would've been a good pick since user rights above rollback do need commitment.

I did consider setting an age barrier for admins, primarily because I needed to add explicit words into the Discussions Abuse Filter. I even considered it for Thread-Mods too because of a similar thing; they'll eventually be looking at posts with explicit language or images, and learn how to deal with it.

I guess responsibility and decision-making comes first though, and enforcement of proof-of-age policies on FANDOM is pretty much impossible AFAIK, apart from the "slip up with a comment accidentally revealing your age".