Maybe never. Many adults collect porceline dolls or barbies or other toys because they never got too old. Maybe at age ten or so you shouldnt do it in front of your friends tho. Just in your house or your room.
According to some research sources, one is supposed to gain interest in playing with dolls when he or she is about 7 months old. However, this can happen earlier than that or later and can go on till the person is about 14 to 17 years though some still continue with this habit.