Last season? We saw them turning a baby into a blue eyed thing seasons ago, but are not all of the white walkers dead? If they are dead, how do they age / grow?
My understanding, (and granted, this involves a lot of conjecture on my part), is as follows...
In order to create more Others, the Others first need human male babies. How they get from babies to adult looking Others involves some kind of magical transformation. They are not made into Other-babies who gradually grow to full sized Others. I would guess that we have 1 of 2 scenarios.
1). A magical transformation occurs and the human baby is transformed into an Other.
2). Something else is transformed into an Other (could be a dead human, a snowman, a tree, or something else) but a human baby has to be sacrificed in order for the transformation process to occur.
If you get deep into the discussion of the books, many fans believe the following theory.
-at some point in the distant past, the Starks had a truce with The Others, which required the Starks to sacrifice certain male babies (maybe bastards?) to the Others. This was accomplished via taking the babies through the Black Gate (the gate that Sam and Gilly used to return from north of the Wall and Bran, Meera, etc used to pass beyond the Wall).
-The Night's Watch was aware of this agreement and was complicit. They helped the transactions occur.
-Over time, the baby sacrifices stopped happening due to TNW and the Starks forgetting they were supposed to be doing this.
-Craster has Stark blood and was perfectly happy to give his sons away, which prevented the Others from getting too pissed off that the truce terms were not being upheld.
-However, they eventually got so pissed off that they decided they were no longer bound by the terms and decided they would destroy all mankind.
-Now, one of their main objectives is to take revenge on the Starks.
