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
While the E1 and E2 Kryptonians may have had the same powers, they were genetically different. The E2 Kryptonians had powers on Krypton (around Action #1 levels IIRC) and it took them a bit longer to reach their full potential on Earth.
As for E1 Kryptonians aging, there is the theory that they only aged because they wanted to. This theory is backed up by this Cary Bates story:
"Hypnotists use the power of suggestion to make subjects believe what they want them to -- But with my super-power of suggestion -- my mind can make whatever I believe has happened really happen!"
-- Superboy's/Superman's thoughts, AC #466, page 16, panel 5.