This is a difficult question, because the Bible does not directly speak about this issue. Some people believe that whatever age you are at death, that is the age you will be in heaven. Others say that we will all be an "ideal age" in heaven, perhaps early 30s, the prime of life and the age of Jesus at the crucifixion. The Bible does not directly answer this question. However, if we piece together a few things that we know to be true, we can come up with a reasonable answer.
In heaven, what age will everyone be?
Heaven is an eternal place (Daniel 4:34). It isn't natural for us to think of ourselves living forever and never aging. We exist within linear time; our existence is linear, moving from point A to point B. Part of the reality of that linear existence is the process from birth to death. But birth, death, and aging are not a reality in heaven. In God's kingdom, we will live forever, and the idea of aging will likely have no meaning, or at least a very different meaning. Perhaps we will understand that time is passing, but it will have a different effect on our bodies and minds. There is really no way to know until we're there.
We do know that in heaven, we will have a "new body" which God calls our "eternal house" (2 Corinthians 5:1). The bodies we will receive will be entirely new, and will replace our earthly body—in fact, God says that this new body will "swallow up" what is mortal about us. It will be a heavenly body that does not die, age, suffer or sin (Revelation 21:4). Still, it makes us wonder: what do those bodies look like? The Bible doesn't say.
When we wonder about our age or appearance in heaven, it is usually because we want to know if we will recognize those we knew during our lives. If our heavenly bodies are new, there is a chance we will not look like we did here on earth. If that is so, how will we know each other? The Bible doesn't tell us this either. What we can be sure of, however, is that our existence in Heaven will surpass our existence here in every possible way. The depths of love and purity and joy that we will know in God's presence will be astonishing (Psalm 16:11).
So, we know that a) heaven is eternal, b) we will have new bodies that are better than our mortal ones ever were and c) we will be in God's presence, experiencing fullness of joy and pleasure that is beyond anything we can imagine. It seems most rational to assume that we will have no age at all, and that if we do have an age, it will be the last thing on our minds.
Is heaven real?
In heaven, will we have physical bodies?
What will we be doing in heaven? Won't it get boring if we are there forever?
When we're in heaven will we remember our earthly lives?
Will we know our friends and family members in Heaven?
Truth about Eternity