Let:
a- years of an older boy
b- year of yanger boy
Prescolar age is up to 6-7 years, well it even could be 5 years.
Thus product of a*b could be any number from 1 (1*1) to 49 (7*7).
N-number of birds on fence.
Additional information as per DS question is:
1) smaller boy is similar to me
2) N does not equal to 0.


We know that a>b, and a*b=N.
Considering that birds are on fence, thus neither a , nor b equals to 0.
Put yourself in shoes of the "A" guy, who counted birds. He counted the number of birds, and than he compared that number to a product of any two numbers. In his mind, N is formed by some different pairs of figures: let say (a1;b1), (a2;b2), (a3;b3) -the combination of possible values is limited. (for example he counted 12 birds on fence, so possible sollutions are only two pairs 2;6, 3;4, and not 1;12. - but in this case there is no clue what are the boys' ages). After he understood that one of child is smaller, THIS FACT was sufficient for him to conclude about the ages.
So, in his possible solutions (pairs of figures) were a pair of two equal numbers and a pair of non-equal numbers. Factor of two equal numbers is a perfect square. Perfects squares are 4,9,16,25,36,49 - up to 7.

Possible pairs are:
4- (2;2) and (4;1)
9- (3;3) and (9;1)
16-(4;4), (1;16), (2;8)
From these pairs only first pair of figures has prescolar ages. So boys' ages are 4 and 1.