The narrator knew the number of the man’s house, so the only way that this clue didn’t help him is if the sum is 13, in which case it’s still ambiguous. So we know the ages are either 1, 6, and 6, or 2, 2, and 9.
The third clue makes a reference to the oldest daughter, so the ages must be 2, 2, and 9.
(It’s true that there could still be an “oldest” daughter in the case of 1, 6, and 6, since they could be 10 months apart and both be 6. But since the man gave this as a clue, we can assume that it’s suppose to push us towards the 2, 2, and 9 solution.)