Literally, the boss could be any age I have issue with your use of "literally", but I'm willing to put a pin in that.

Generally, at 18 years old, you've not had any significant amount of time in managing people. You lack the experience to understand where the other workers are coming from and therefore any empathy you have for them is constructed from an inexperienced world view. While not impossible to be a natural born leader at 18, most are emotionally and intellectually ill-equipped to handle day to day problems at arise on the workplace. In context, this 18 year old was placed into this position, not because of their natural born leadership and excellent negotiation tactics, but rather because the family needs to groom them to take over the business. In my decidedly greater than 18 year old experience, those situations do not breed strong leaders and those that are under them generally suffer. In your late 20's to early 30's, given the appropriate opportunities to work in management, many can thrive. It's not ageism, it's just that maturity and experience does play a significant role in being able to lead others, especially when it comes to others that aren't particularly good at taking orders or producing work without close supervision.