Why do us adults get shocked or upset when kids grow up so fast?

My friend's nephew turned 4 yesterday and her sister was like, "How the heck is he 4 already! If these kids don't stop growing up I swear to god!!!"
"Her grandpa says, "My grandson is growing up so fast! Stop it! Lol"
When my little cousin turned 12 I was like, "When the hell did this happen!" Why do kids insist making is feel old?! Why can't they stay small forever?! 😭 lol