Older man dating a younger woman is still culturally accepted but not vice versa. Statistically, this means that the dating pool for older men just keeps increasing.

For older women, the situation is the exact opposite. Their dating pool, which consists of (say) all older men keeps decreasing with age.

That might explain why your older female friends find relationships harder as they age, rather than a decrease in their physical attractiveness. The latter is what many women attribute their situation to, but IMO its just statistics and preferences.