Consider:
Childhood never comes again in life.
What might be single word that means never come again in life?
Consider:
Childhood never comes again in life.
What might be single word that means never come again in life?
The expression once-in-a-lifetime may fit:
Describes an experience or opportunity that is very special because you will probably only have it once : A tour of Australia is a once-in-a-lifetime experience .
A chance that will never occur again in one's lifetime. This is a once-in-a-lifetime chance. Don't miss it. She offered me a once-in-a-lifetime opportunity, but I turned it down.
There is one-off in BrE.
1. adj. Happening, done, or made only once.
2. n. Something that is not repeated or reproduced.
Example: (from the book "Global Citizenship and the Legacy of Empire: Marketing Development" By April Biccum)
Childhood is a one-off window of development, which makes investing in children a priority but not a choice. Damage at this stage cannot be overcome later in life. Northern child poverty is the root of the most serious social problem facing these countries in the future.
(Wilkinson, 2000, p. 17, my emphasis)