The Golden Age is the unofficial name for a principal/law that is applied to the souls of the dead in the afterlife.
Overview[]
When a human dies the Valkyries will guide their soul to Heaven. Once the souls of the dead arrive in the afterlife, they will take the physical form that they had during their Golden Age, the age in which they were at their peak.
This means that their physical appearance have nothing to do with their state at death.
The Golden Age refers to a person's peak, however this is not limited to only the peak of their physical abilities, other skills and aspects are considered when determining the Golden Age of a person. However, due to the health problems present during one's old age, most humans in heaven have a healthy and youthful appearance, usually somewhere around their Adulthood.
Aging in the Afterlife[]
While rare it is possible for a person to age in the afterlife. In order for this to happen they need to improve themselves to the point that what can be considered their Golden Age changes. The only known exemple of this happening is Sasaki Kojiro, who after training for over 400 years has aged considerably, going from an adult (who died in his thirties) to an elderly man.
Animals and their Golden Age[]
While not directly confirm, it can be assumed that the Golden Age law is also applied to animals. This can be assumed, since Red Hare possessed a healthy body even after being executed on Earth.