A man gets to know his neighbor, and he learns that she has 3 girls. Intrigued, he asks for their age. The lady, who doesn't want to give him this information straight away, has him to take a guess.
Lady : The product of their ages is 36.
Man : I definitely need more clues...
L : The sum is equal to the number of the house across the street.
M : OK, so, you're not helping, uh?
L : The elder one is fair-haired.
M : ... Oh! I know!
How old are the three girls, and how could the man guess it?
Not sure about my english and/or the tags (first post here, hope it is not a duplicate), so feel free to edit.