That's because fire giants are human-looking so one expects the same kind of life cycle from a fire giant as one would from a human. That would create all kinds of baggage about what it means to be a "baby" as you put it. However, there's no such thing when we're talking about the life cycle of non-humanoid, extraplanar, creatures. For all we know, salamanders are 100 years old, sprung forth from the plane of fire, waiting to one day grow to their full potential.
I mean, even if they were 1 month old when you fight them, if they are created with a full sentient and alien mind to that of (demi)humanity, then why are we using human standards to relate to them? Would not that make as much sense as referring to a "baby" Cthulhu? Does referring to a even newly formed monstrosity from another world as a "baby" or "infant" or "toddler" even make one bit of sense? What if that dretch just came into existence 5 minutes ago? Do we not kill it because we saw it spring forth from its larval cocoon?
If the fire snake never turned into a salamander, would you care how old it was? Would killing the week old salamander really bother you? Is the distinction here because it isn't fully formed so there is some inherent wrongness to a battle with it until it reaches maturity?