Assuming whole years only, possible permutations are:
- 1 + 1 + 36 = 38
- 1 + 2 + 18 = 21
- 1 + 3 + 12 = 16
- 1 + 6 + 6 = 13
- 2 + 2 + 9 =13
- 2 + 3 + 6 = 11
- 3 + 3 + 4 = 10
As has been stated, the only way he would still need further clarification when given a house number would be if the sum of more than one of the permutations was the same. Even though that falls on thirteen, and its incredibly rare for a house to bear that number, it is the only alternative and possibly chosen to cause us to second guess our premises.
The result is that we must have twins, but the fact that the eldest plays the piano would indicate that it must be clear that the eldest is identifiable from the riddle, because the numbers bear little relevance to piano plaing or musical scales. Hence I suspect the mention of the piano is to further throw us off the scent.
My choice would thus be 2, 2, and 9.