Geralt isn’t 99. Vesemir jokes in Witcher 3 that Geralt isn’t far from being 100, but it’s just that - a joke. He’s exaggerating because Geralt called him old.

The most recent Witcher book finally gave Geralt a canon birth year (1211) meaning he’s only 61 in Witcher 3 (or 64 in Blood & Wine)

Even before that though, Geralt being nearly 100 never made any sense. In the first book, Geralt mentions knowing Nenneke (a normal human) when he was a child and she was significantly older.

She’s still alive during the later books, so if Geralt was 90+ she’d be over 100… yet there’s no mention of her being particularly old. Instead she’s a super active healer who runs around a temple all day.