An ageing spell allows the caster to age themselves so that they appear physically older. Ageing spells can be considered advanced forms of another kind of spell, the glamour, and are notoriously difficult and unpredictable. It's not clear if the warlock or witch can control the spell and decide how much to get older. When Merlin used this spell, he turned into an 80-year-old man, declaring before that it was his intention to get this old.