Hussar
Legend
Your average to above average 1e 20th level fighter had 82.5 hit points. If he was lucky enough to be well above average and have a con bonus, he probably didn't have more than +2 per hit die (not level) for a 16 con and had 100.5 at 20th level. If beyond all expectations he was among the very, very few to have an 18 con, he had 118.5 hit points at 20th level.
Your average 20th level 5e fighter will have double that in all probability. The other classes couldn't even get more than +2 per die, had lower hit die types, and less hit points per level after 9th, so they won't even have that 100 that the fighter has. For example, a 20th level thief with no con bonus has an average of 53.5 hit points at 20th and an average of 71.5 if he has a 16+ for con.
This REALLY depends on rolling method. If you used the method in the Unearthed Arcana, for example, you rolled 8d6, choose the best 3 for your Con. Having an 18 Con wasn't that much of a stretch.
But, in any case, I was thinking more about single digit level characters. Sure, after 10th, the 5e characters will race ahead, but, prior to 10th, there isn't much of a difference. A 5th level AD&D fighter or a 5e fighter is somewhere around 35 HP, by and large.