My cat is 11 months old and he’s started shedding a lot

So my cat is 11 months old and he’s started shedding a lot. Right now it’s winter and it’s cold outside but he lives only indoors, it’s warm indoors.

My question is: is he shedding because it’s winter (even though he lives indoors) or because he’a changing his kitten fur to a grown up cat fur?