This is the adult price.
There are only three adults.
The adults went to an izakaya.
This kanji combines big and person. If you're a child, how do you think of adults? You just think of them like big people. That's why this combination of kanji means adult.
"I'm an adult, which means I have adult responsibilities. Oh, Toner (おとな)? I need some of that for my terrible printer..." ... now feel so sad you have such a terrible printer that runs out of toner every day. Ohhhh Toner!