Of course it's a real thing - and it may also be at the same time "numb lives catching up to them". Or the realization that your time on earth starts running out. Or the effect time has on your body - you just cannot do many things as easily as you used to. Often your kids are leaving home around this time, too, causing a sudden change of how you are living and making you think your future anew now that a responsibility has gone. Sex drive goes down too, so your relationship with your significant other changes.
Of course it isn't always the situation of "I am young, look at my sporty sports car, and how jive I still am" or trying to find younger women interested in you to boost your self-esteem. It may come as a slight depression in general. It may show itself in trying to do new things, learn stuff. It also doesn't affect everyone. But it is a real thing.