Radio Astronomy Systems Research Group

The sons are 2, 2 and 9.

Clue 1: The product of there ages is 36.

The friend is very clever, so he comes up with all the following possibilities:

1 x 1 x 36
1 x 2 x 18
1 x 3 x 12
1 x 4 x 9
1 x 6 x 6
2 x 2 x 9
2 x 3 x 6
3 x 3 x 4

Clue 2: The sum of their ages is the same as the friends house number.

Now, we do not now the house number, but that doesn't matter. Of the possibilities above, only two of them add up to the same number.

1 + 6 + 6 = 13
2 + 2 + 9 = 13

Clue 3: The eldest son plays the glass harmonica. (Or was it a theremin? I don't remember)

Of the two possibilities we have now, only one of them has an 'eldest' son. The other has a set of twins that are the same age.