I don't buy it. Some of us have conditions that are out of our control. Those conditions may make exertion something we want to avoid or ought to avoid.

But most of us who dislike exertion don't dislike it for that reason. We fall out of habit which makes the exertion unpleasant. Or maybe we never get into the habit. I think habit is key for most of us. It's best to form the habit when we're young, but that doesn't mean it's too late any any point. If you don't have the habit, you can still form it at any age. That's my theory, and I don't have any science to back it up. I hope a more qualified person will follow up. The truth may be more nuanced.