Here is a good question for you all...
If Superman is invulnerable, then how come he ages?
This bugs me, too... the Earth Two Superman shouldn't look as old as he does, but that was to differenciate him from Kal-El...
I always figured Superman would live for centuries and age very slowly... but then again he aged normally til 18.
On the unified Earth, he still could have been the first hero in 1938 and due to his slow aging still be active today with little difference, looks wise. His girlfriend/wife back then could have been Lana Lang. He could have had a different secret ID (or could now pose as Clark Kent Jr)
Just my thoughts