In the episode "Strange Visitor (from Another Planet)", Jonathan tells Clark that they found him in Schuster's Field on May 17, 1966. In "Never on Sunday" Clark says that his birthday is February 28, 1966. I read on one of the boards somewhere that Clark's birthday has historically been February 29 in the comics, but 1966 wasn't a leap year and they wanted Clark to be the same age as Dean.
Now, presumably the Kents couldn't know the exact birthdate, but extrapolated based on how old he appeared to be when they found him. And I would say that the baby we saw in "Tempus Fugitive" looked anywhere from 2-5 months old...
Kathy
