What is the ruling on an imam who leads prayers with tayammum due to old age, even though warm water is available?

Old age is not an excuse that permits tayammum. If the aforementioned Imam is not harmed by using water and is capable of performing wudu’, then his tayammum and prayer are invalid, as is the prayer of anyone who knows his condition. As for someone who is ignorant of his condition, their prayer is valid according to the muftā bihi (the relied-upon opinion), though it is more cautious for them to repeat it.