I am aware of the Adult Jobs for Teens mod by sparklymari, but that seems to just individually make all the jobs in the game also available to teens. The problem is that custom careers I've added into the game are then not available to teens anyway, only the in-game careers are. Is there any mod that just categorically unlocks all adult jobs for teens, so that custom careers also get included?

Replying to myself with my own solution in case anyone else ever searches for this.

The way I made it work was by editing the custom careers myself. I opened them in Sims 4 Studio and just went through everything, looking for all the places where ages were listed (most would be Young Adult to Elder) and simply added Teen to the list in all those places. There are some places where ages are listed in numerical values so that's something to keep an eye out for, I think it's 8 for Teen, 16 for Young Adult, 32 for Adult, etc. It's a bit tedious to go through all the code, but it works! Keep in mind that if you update the mod later, you'll overwrite your own changes and need to do it again.