Theogonies (Angel Exterminatus Spoilers)

I have recently been reading Horus Heresy: Angel Exterminatus by the excellent Graham McNeill.  I don't want to tell you anything about the actual story itself.  But I wanted to discuss a few parts of the novel here.

Specifically, there are three (sub) chapters entitled "Theogonies".  Within those chapters, we read and learn of the earliest days of Perturabo, Ferrus Manus, and Fulgrim. Of these primarchs, there are clearly variations in their histories and their planet of landing. But what really struck me about the early histories are the age of the primarchs upon landing.

Fulgrim is still a baby essentially and lands as such before being discovered.  Ferrus Manus on the other hand is a fully grown youth (teenager equivalent perhaps?) when he steps out of his pod and looks around his environment.  Perturabo is a bit more ambiguous as we pick his story up during an ascent of a cliff -- it is hinted he doesn't quite know how he got there. So we can't draw too many conclusions there.  But why should the newly landed primarchs have such different ages / maturities?  Was is that their pods took different times to fly through the warp?  Do their ages relate to the distances of the planets (Medusa, etc.) away from Terra?  Or did they genuinely age differently?

But even more puzzling to me is this: why did most of the primarchs have their names given to them, whereas Perturabo remembers what his name really is?  Who named Perturabo?  Was it the Emperor, or did he decide himself?  An entertaining conundrum!