Ageing is more brutal for male lions than for females.
Female lions live in natal prides, & in old age there are always sisters, aunts, & daughters around to care for them and share food.
Male lions, however, often become solitary nomads with no one to look after them.
Male lions don't remain king of the pride for life. As they age, younger & stronger coalitions challenge & eventually push them out.
With no pride of lionesses to support & hunt for them, a displaced male must hunt alone & also defend himself against rival territorial males.
With age, teeth wear down & joints deteriorate, & hunting efficiency declines steadily. Other predators, particularly hyenas, notice and exploit this.
Any prey a weakened lion does manage to catch is often stolen by scavengers & competitors that would never have dared challenge him in his prime.
It is a complete role reversal. At his peak, a male lion stole kills from other predators without hesitation. In old age, the situation flips entirely.
Unable to hunt effectively, the male becomes a scavenger, eating only what others leave behind. Starvation sets in. The body loses fat, then muscle.
Life comes full circle: The killer becomes a victim themselves. The younger lions & other predators become a threat to old lions.