That depends on the manager. Some of them get it because they know someone up top, some get it because they have a degree and know real-world experience, and some young managers are just well-qualified. I've held several management positions now, and I'm under 30.
More often than not, I'd say it's the degree thing. Getting a diploma doesn't make you a good manager, and it only teaches you theory.
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Hi JG11Bravo, That's true. Plus I think they should have some practical experience before they leave college in the role. That way they know what is expected. The more experience you have the better you become.