You're not supposed to actually know that the license plate starts in 13. That is a clue for the neighbor trying to figure it out. Think about it critically. If the neighbor can look at the car and see the first two digits, he knows the answer immediately unless those first two numbers happen to be a 1 and a 3 because there are two different combinations of ages that result in a 13. If it was anything other than 13, he would not need the added information that the eldest daughter liked strawberry ice cream. It would be worthless information. You, as someone looking at this riddle from the outside, don't need to realize anything other than the fact that the first two digits on the license plate did not help the neighbor solve the riddle at all. When you see that he wasn't able to solve it, you then realize that the only possibility is that it is one of the two combinations of numbers that equal 13. Then the ice cream clue about the eldest daughter and, bingo-bango-bongo, you can figure it must be the 9-2-2 answer.

No clairvoyance, lucky charms, or preexisting knowledge of one's mileage (this one I really don't get) are necessary at all.