Clue one gives all possible combinations of three ages that multiply to 36, which are:

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

Clue two still doesn't give the insurance person enough information, meaning that there must be (at least) two combinations adding to the same number. This leaves us with the two combinations:

2 2 9
1 6 6

which both add to 13.

Then last clue narrows it down to 2 2 9 as this is the only combination with an eldest.