How do companies and businesses get away with charging adult prices for people over some arbitrarily assigned age?

Last time I checked a person was a child until they hit 18, this is the law.
At 18 they are considered an adult, not before.

So why is it companies when selling products, tickets for events, hotel rooms etc, are freely allowed to redefine what is already set by law in order to charge more money?

I’m not aware that a business can change the law off it’s own back and lower the age of buying alcohol or cigs for smoking, just on a whim that they see it as being “fairer to their business”, so why is a 16 year old charged adult prices in hotels, at concerts, on various transport methods etc… without the rights of an adult?