I clicked on the Instagram account of a male friend to check out photos of his new girlfriend. My friend is pushing fifty, good looking, and a successful professional, and I was expecting his girlfriend to be attractive and young.

I was wrong, and I was shocked, though pleasantly so. His new girlfriend was clearly older than him. And my surprise reflected how unusual this situation is. Middle aged men frequently date women younger than themselves.

Michael Douglas and Catherine Zeta-Jones have an age difference of 25 years. Credit:AP

I've seen this play out repeatedly in my own life. Most of the fifty-something men I know have, at some point, been involved with women in their thirties. It's so common it's a cliché. And the relationships go one of two ways. Either they end in heartbreak, because the younger woman wants babies and the man can't bear the thought of starting over, or they stay together, and the man eventually becomes a father again in midlife.

So why do so many older men hook up with younger woman? Well, the obvious answer is 'because they can'.