He's not against using old codgers if He has to. Who knows whether He will in this case? It doesn't matter. Like Elijah, and Moses, and all those other prophets, they will have the energy to do what they need to do for the 3 ½ years that they are supposed to do it. So here we have our first pairing - Moses and Aaron. They function as leader and spokesman, much like God the Father and God the Son. Later they were paired as prophet and high priest, and they fulfilled those jobs.
Moses was in the position of being God to Aaron, even as God was God to Moses, and Moses was God's prophet, but Aaron was Moses' prophet. A prophet is simply one who speaks for another (speaks the words that the other put into his mouth)—a very simple arrangement, easy to understand.
