Do they ever mention it in the show and if not what do people normally assume? Middle school, High school, younger?

I don't think it was ever specifically mentioned. I don't remember which episode it was in (one of the Christmas ones maybe) where she mentioned seeing her father kissing the neighbor lady and I think she mentioned she was nine.

In Lucky Leon during their date Lois says:

It doesn't mention specifically that her parents were living together at the time. It's possible that Lois moved in with her dad due to some altercation with drunk Ellen.

I was just reminded that Lois mentioned in Season's Greedings that she was 12 when her parents split up. She broke into her piggy bank to buy a tree.

Thanks, Ann.

I was thinking she was 11 or 12.