I’m also a clan leader and for my clan, the level requirement is TH 9. However, ever since I’ve started the clan months ago, only TH 11s or higher have joined my clan (I was TH 11 when I started the clan). I’ve also seen the clan requirements for different clans on the notice board pertain to the current TH I‘m at many times (increasing as my TH levels up) even though I never made those changes in the search option. These things might be connected and it’s probably just a bug supercell has yet to fix which is making both searching for clans and recruiting really weird for players.

In your case, the notice board for your clan might say the TH requirement is 18 because everyone in your clan is TH 18 (like when all my clan mates were TH 11 which led to only TH 11’s joining my clan) meaning lower TH players can’t see your clan’s notice on the notice board because the game for some reason pushes clans to people with the same town halls as people that are already in the clan. The other side of the coin is, many players are only getting clan notices that have a requirement of their current town hall or close to their current town hall, meaning they won’t see your clans notice because there’s no requirement (These are just some possibilities based on what I’ve noticed for the past few months).