Nowadays, age means nothing. Due to the school system, children standardly grow up with friends either in the same grade, or one grade higher or lower. If one’s sibling is a few years apart, they might socialize with their sibling’s friends, but that’s the extent of it.
Married couples are often multiple years apart, but at that age it’s not considered odd. At a younger age, however, it would be weird if people were to date even three years up. The age gap of friendships/relationships is a double standard, and it’s simply weird to reflect on how that evolves with age.
More controversially, milestones that come with age seem to be happening at younger and younger ages. Only a few years after reaching double digits, children are drinking and doing drugs. Maturing at such a young age leaves nothing for these adolescents to do at later points in their lives. This premature maturing is detrimental to the children, and it messes with cultural norms.