this may be off a month or so but a rule of thumb to go by is... at 2 they cut their first 2 perm teeth. at 3, 2 more, at 4, 2 more and at 5, the last 2. this is how you age them. after 5 years of age, it becomes a guess, based on wear.
so at 5, or 4.5, whenever they cut their last 2 teeth...