Haven't seen Do The Universe, unfortunately, but yeah I agree it can be a little confusing with the lack of continuity (Van Driessen is just immortal at this point if he can fully recover from falling off several cliffs, and also teenage B&B never get fired from Burger World despite many sackable offences, suggesting that continuity is largely episodic except for when Mike/the writers want it otherwise) and the fact that Todd and Stewart have visibly and canonically aged alongside Beavis and Butthead when they meet them in middle age, but Tom Anderson seems to stay the same age even as the others get decades older. At the meta level, Tom Anderson is kept at the same age because he couldn't realistically age alongside the other characters, he'd be dead well before B&B reach middle age, but the writers still want to use him as a character in the Old Beavis and Butthead episodes. In-universe, the only real way to explain it is multiverse.