In prayer, the imam should be a qualified person. If the group is mixed men and women, the imam must be a man. The qualifications of the imam are elaborated in the Sunnah of the Prophet (peace and blessings be upon him) and clarified in books of jurisprudence.
Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states: “It is good to pray in jama`ah (congregation). Wherever there are two or more Muslims together, they should pray in jama`ah. The imam should be an adult if he is leading the prayers of adults. Minors [i.e., those who have not reached puberty] are not allowed to lead obligatory prayers. They can lead voluntary prayers if they know about purification and can read the Qur’an well. It is good to ask young children of seven years or older to lead voluntary prayers sometimes to encourage and teach them how to lead prayers.
When praying in jama`ah at someone’s home, the owner of the house has the right to lead the prayer, if he knows how to do it. The lady of the house can also lead the prayer if there are only ladies praying together. The outsiders should not lead prayer in someone’s home without his permission.
Generally speaking, the person who knows the Qur’an more should be given preference in leading prayer. If they are all almost equal in their knowledge of the Qur’an, then the person who knows more about the Sunnah should be given preference. Otherwise, the most senior person can be chosen to lead the prayer. These are recommendations only. It is permissible for a less knowledgeable (even less pious) person to lead in prayer those who are more knowledgeable (or more pious) than him, as long as he knows the basic rules and method of prayer. The following hadith of the Prophet (peace and blessings be upon him) gave the guidelines in this regard:
Abu Mas`ud Al-Ansari reported that the Prophet (peace and blessings be upon him) said, “The person who can best read the Book of Allah will lead the people (in prayer). If they are equal in reading (the Qur’an), then the one who has more knowledge of the Sunnah. If they are equal in the knowledge of Sunnah, then the one who was first in making the Hijrah (from Makkah to Madinah). If they are equal in that then the one who is more senior (or older) in Islam. Let not a person lead the prayer in the place of the authority of another person and let him not sit in his house in his place of honor, except by his permission.” (Reported by Muslim)”