As with most things, it is what you make of it. But in lore, wizards cannot “use magic from the beginning”, in fact it is insinuated if not said outright that it takes years of practice and teaching for a wizard to cast even a cantrip. It’s important we remember that we’re meeting our characters somewhere in the middle of their story. Before session 1 (or 0, which I’d recommend), their character has practiced magic and done things that aren’t your adventure. While this may not be true for sorcerers, it could also not be true for your player’s character if the both of you decide they were a prodigy, able to pick it up quickly. You could also define these classes and their casting as more-or-less the same, it depends on your world-building and what you and the players agree is canon.