The ecclesiastical leader over a potential missionary (usually a Bishop) meets with the young man or women to determine if they are worthy and have a desire to serve. The Bishop will encourage the would be missionary to work and save as much as possible and the family would be asked to help where the potential missionary falls short. There are other options the bishop has to help the missionary, such as members of the local 'ward' (um maybe in other religions it would be called a parish?) and then there is a church wide missionary fund that can help. Usually the bishop will meet with the youth soon enough before to give him plenty of time (years) to prepare, finacially and spiritually. So the answer is No. There isn't a specific age but the Goal of the Church of Jesus Christ of Latter Day Saints is to help Our heavenly fathers children (that would be all of us) to return to Live with him again, as families. So the teaching of the Church are pointed towards preparing us to make certian covenants in the temples and to prepare us to serve others both members of our church and all our Heavenly Fathers children all over the world. And to make it short, the preperation starts as early as we can understand and continues until we are in the grave.