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, and when used like an adjective, it means mature.
"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!