Naomi,

>He could have asked: do you have a twin, but then this puzzle would loose tremendeously and will be very simple.

The two 15 year olds don't have to be twins.

>I solved such problems before and they all have this kind of the third question.

In this puzzle it's just not a valid question. The oldest 15 year old or the 25 year old person would have answered that question yes.

But it's probably only some backwoods place where a 15 year old could be the oldest person living in a house. *g* If this census had been taken in more recent years social services would have shown up the next day. *L*