Need a root canal and probably a crown at 21 years old - advice?

Hello there, don't wait to get the molar restored and treated, you can save it!. When a root canal is done, the filling or material placed where the missing tooth is, is not sufficient to provide enough strength to resist biting forces, I think that's why the previous molar fractured, when you don't protect it right away, it can fracture and you end up losing all the work completed. So don't wait for this to happen to the other tooth, take care of it and restore it with a crown right away. I had done crowns on 18 and 19 year olds, to prevent losing molars at a very early stage.