He has enough wear on the 2nd molar to go with 4 1/2. The question is the wear on the 3rd and last molar. The tips show wear that makes the dentine look as wide or wider than the enamel, but you don't use the wear on the tips to age. It is the slopes leading to the tips that are the marker. In addition, the small third cusp at the very back of the 3rd molar does not show the hard angled wear that would be characteristic of a 5 1/2 year-old. That's why I stopped at 4 1/2.