Up to a point you're right about the joints getting stiff as we age but it's also due to the fact we don't have hinges in our shoulders. Arms above head always have to include some rotation, else you can't do it. The ball joint would pop out of the socket. Speaking from experience, I can honestly say that dislocating your shoulder is a lot less fun than it sounds.

Coppula eam se non posit acceptera jocularum.

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