Any age as long as you know how to sing. In situations where the duet member has not attained the age of entering into a binding legal agreement, their parents will act as the child's legal representatives. A good example is Aaron Dismuke, who provided the voice for Alphonse Elric was born in 1992.
Do you have any youtube videos?? Id like to hear you sing lol...im 13 and sing myself ive been in 5 competitions against people that have sung for like 20 years and come in2 nd in all of them ....id like to be on duets but dont know how to contact them :/ lol so hit me up :)