The dutch man talks about age discrimination on Tinder; I imagine that only in recent times has record keeping gotten so omnipresent, and that people have always had more margin to play with their age +/- a few years.

And yet we cannot lie about it much for long. As if there's a hidden clock somewhere, related to rotating around the sun but barely not. Perhaps those in the future will develop a biological clock, but then we go back to the old question of whether this dutch man is wanted less because of age. Is age just the number on his government ID card?

Why can one fudge age, but only a little?

In Argentina everyone has a national ID, that in particular is used to vote. Until ~1950 only the men had it, and they got it during the conscription.

In ~1950 the government gave all the women also an ID to vote, and the age was self reported because the official records were a mess (most of the records were only the baptism date keep by the church).

Many of the old women subtracted 5 or 10 years from their real age :).

Strictly speaking, we have that already — I forget the English term but a court can declare you to be childlike and appoint a guardian to take care of adult affairs for you.

But that's not what you meant, right? You meant: what benefits would we get from having more complex "age" machinery in every database? Well, the complexity can be an advantage in itself. Much employment for highly paid database specialists, and many (small?) opportunities for blackhat mischief by leveraging people's assumptions that one "age" is the other "age".

"What would happen if, as with gender, we bifurcated the terms for physical vs mental age?"

The size of the electorate would shrink sharply.