No one knows the true origin of the idea but there's a number of examples that predate how I met your mother by over a hundred years.

The first published example is Max O'Rell in Her Royal Highness, Woman: And His Majesty—Cupid from 1901

I heard the other day a very good piece of advice, which I should like to repeat here, as I endorse it thoroughly : A man should marry a woman half his age, plus seven. Try it at whatever age you like, and you will find it works very well, taking for granted all the while that, after all, a man as well as a woman is the age that he looks and feels.