This is a tricky one to answer because the association of youth with life and old with death is not an absolute truth in our own society, much less across multiple societies.
Accordingly, there will be no answer which definitively pins youth to life and old age with death. However, there is a strong tendency to make this correlation. We can discuss why this pattern shows up again and again in cultures, we just won't arrive at the conclusion that it is a universal.
I think one of the most pronounced reasons for associating life with younger age is that one of the defining characteristics of life is its potential. Things that are living have the potential to to great vibrant things. A seed has a potential to become a great tree, while the tree is already what it is going to be.
We can see parallels to this idea in the Chinese concept of xue-qi. If I may grossly simplify the translations, xue is the Chinese word for blood, and Qi is their word for one's life energy:
The [morally] noble man guards himself against 3 things. When he is
young, his xue–qi has not yet stabilized, so he guards himself against
sexual passion. When he reaches his prime, his xue–qi is not easily
subdued, so he guards himself against combativeness. When he reaches
old age, his xue–qi is already depleted, so he guards himself against
acquisitiveness. - Confucius, Analects, 16:7
In this, we see a pattern of starting with potential and ending with depletion which strongly correlates to the idea of "life" and "death" as you refer to them in your question.
As a general rule, the young are more likely to still have great potential in them, simply due to having more time and having a body that hasn't worn out yet. There are absolutely counter examples in the world, but the general rule is strong enough to create a general pattern of associating youth with life and old age with death.
As for the idea of life coming from death, as you mention, this does indeed occur in our lives. It's why we have wakes to celebrate people's lives rather than funerals. It's why we appreciate what a forest fire does to revitalize a Forrest. However, these are generally more difficult concepts to work with. It is much simpler to draw the connection between youth and life.
And finally, the answer may simply be "because it works." Cultural patterns like this often form because they worked well for many generations. Perhaps it helped with the grieving process. Perhaps it spurred youth into action while they still had the bodies to act. Never forget that cultures are living breathing organisms in and of themselves.