Let's also remember that it's a comic book and certain rules of physics are bent. It's suspension of disbelief. Whether Superman's matrix was sent through a wormhole or went faster than the speed of light (which is clearly possible in the DC Universe, hasn't Flash done it?)
As I remember it, the current comics have Krypton exploding thousands of years ago, and Supes embryo was in the matrix. He wasn't technically 'born' until he neared Earth. So he COULD have been traveling for thousands of years in a suspended state. As far as I know, Superman's 'birthdate' can basically be equated to when he arrived on Earth. I think he is perpetually meant to be around 35. When Doomsday killed him, it had been 10 years since Superman appeared in Metropolis. So basically, I have no answer

Believe what you will, but I think mid-30s is a safe bet.
http://www.gsd.harvard.edu/~nrollins 'Strange things are afoot at the Circle K.'