In short: as soon as they express more than a passing interest. If your child is tapping away on their leg whenever they hear a catchy rhythm, perhaps it would be a good idea to see if they enjoy transferring this to drumsticks and one of the kits listed in this guide. For some kids this might be age four, others age 12. It really depends on when they’re inspired and what inspires them.
One scroll through YouTube will show you that there are kids around the world, of all different ages, who have a passion for the drums. What this highlights is that there is no “right age” per-se, to begin learning the drums. Only that when a child does show a level of interest, it would be a very good idea to nurture this as early as possible.