Never mind, I fixed the issue. For anyone wondering it was caused by an outdated mod.
But why can I not do it with sims older than teens?
@chewiicheeze As part of its T rating, Sims restricts romantic behaviour between adults and non-adults.
For example young-adults, can't interact with other young-adults they have a romantic relationship.
@chewiicheeze Okay, so you're going to need to test this with no mods or CC installed, then. Don't include Teen-nonteen relationships in your testing.
I have, and I know it's a problem with one of my mods, and I have tried taking out larger mods, or ones on the list of broken ones with the new patch, but I'm still having the issue. (I have also updated to the March 7th version of the game and the issue is still present)
@chewiicheeze Unfortunately, you may need to do a full 50-50 . As part of the testing process, check installation instructions.
