God's age is not known. The Bible says: "From everlasting to everlasting you are God." And Peter wrote: "But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day." (2 Peter 3: 8). In heaven time will be no more.
Moses and Elijah appeared in a special moment with Jesus. Peter, James and John were there. God spoke to them from the sky. Both Moses and Elijah had been dead for 1,000 years. Today they would be 3,000 years old.
Wow, that's a lot of birthdays!!! (See Matthew 17: 1-9)
Please note: The disciples knew who Moses and Elijah were. How, I know not but they looked pretty normal, so God's message was the major thought: "This is my Son whom I love; with Him I am well pleased. Listen to Him." It is a most interesting time (Jesus' face shone like the sun and the three disciples were terrified.) Coming down from that mountain Jesus said: "Don't tell anyone what you have seen until the Son of Man (Jesus) has been raised from the dead." That's a powerful statement! (Matthew 17:1-9)
Really it does not matter what he looks like. We may not be able to handle it. His purity, love and majesty may frighten us. His voice scared the daylights out of ?The children of Israel? and they begged Moses to let them get away. (Exodus 20:18-21)
To look at him is not permitted. God spoke to Moses face to face because God trusted Moses and at the time God thought it necessary for teaching Israel His laws.
The Disciples asked Jesus the same question as you did. Jesus said: ?If you have seen me you have seen the father.?