It is probably done like this to fit into the lore that witches are mostly described as elderly.in fairytales.
There is reason why it's called spellcaster, not witch and not all witch that old in story, some are young and beautiful in story lore, and some old ugly witch willing to do dark magic to make them young beautiful. hmmm how about mage skeletal rather than elder, a lich!
It is called a spellcaster because they didn't want to limit it to only Witch, Warlock, Alchemist aso. These are by the way also predominantly described as old.
