I see no reason why this toothbrush needs a maximum age recommendation; for example, the adult brush heads can also be used with it and in 'fast' mode the brush has the same frequency as an 'adult brush'.

There is probably no maximum, but there is a difference between kids and adult brushes according to the manufacturer (Philips): "Since children's mouths and teeth are smaller than adults', the deflection angle of the Sonicare for Kids is smaller than that of the Sonicare for adults."
So I would change from the age of 12 onwards with the next purchase.