Question: "What age will everyone be in Heaven?"
Answer: The Bible does not specifically answer this question. Will babies and children who die still be babies / children in Heaven? What about elderly people who die, do they remain elderly in Heaven? Some have guessed that babies are given a resurrection body (1 Corinthians 15:35-49) that is “fast-forwarded” to the “ideal age,” just as those who die at an old age are "re-wound" to the ideal age. This would indicate that there won’t be any children or elderly people in heaven.
What is the ideal age? Again, this concept is not specifically biblical, some believe it to be around 30. Some guess 33 since that is approximately the age Jesus was when He died. First John 3:2 declares, "Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when He appears, we shall be like Him, for we shall see Him as He is."
One thing is for certain. Whatever age we appear to be, we will be gloriously perfect. Our entire person will be remade flawless, wholly and completely Christ-like. We will lose all trace of human fallenness, wearing the white robes of purity, holiness and absolute perfection. So whatever age we are, it will be the age of complete and total perfection.