baby teeth?

at what age do a joeys teeth erupt? isn't it usually when they are oop?

Actually, by the time a joey comes out of pouch, it already has its teeth. At what point during development within the pouch they erupt, I don't know.

are these teeth easily broken? and if so do new ones grow back or they just don't have teeth for the rest of their life?

A glider's teeth are not easily broken, after all - they're used for scraping off bark from trees in the wild

Sugar gliders teeth do not grow back if they break.

When a joey comes oop it has really tiny back and side teeth. The front teeth are of course larger since those are their scraping teeth.
They have very strong teeth but if they break that's it. You would need to take them to a vet asap if you notice any loose, broken, or cracked teeth.