Puppy fur vs. adult fur?

[Rachel the Wild Butterfly]

In a great pyr(guessing it's different for each breed) when does the puppy fur get replaced by the adult fur? My 4 1/2 month old pyr doesn't have hair under his arms(armpit area) and his groin area. Will he get hair in those places? He doesn't have much hair on the bottom of him.

 

[Bonsylar]

I have a Lab and she doesn't have any hair on her tummy or her armpits either.
Same thing with the poodle mix I had when I was little.

I don't know why.