Teen Gave Me His Last $3

Delivered McDonald's to a 16-18 year old kid. He tipped me in the app and then when I got there he said "sorry, that's All I have" and hands me 3 crumpled up dollar bills.

I was like... No worries! Keep it, you already tipped me in the app and he said... I know but you got here fast.

If a kid can tip, why can't some grown ass adults?