Because you are negleting the order of birth. You are counting, for example, "Older boy on Monday, younger boy on Tuesday" and "Younger boy on Monday, older boy on Tuesday" as the same event and they are not.
The correct enumeration of events are as follows:
If older boy is born on tuesday:
7 possibilities of younger girls
7 possibilities of younger boys
If older boy is not born on Tuesday
7 possibilities of younger girls
6 possibilities of younger boys (because you have been told that at least one boy was born on tuesday)
There are 27 total possible events, 13 of which are both boys.