Tropic of Calculus

I've been told, although by whom I can't quite remember, maybe it was myself, that an acceptable calculation for the minimum age of whom you date is half your age plus seven. This is flawed on so many different levels it's almost impossible to begin consdering it, but let's assume that, for a theory with absolutely no basis, it works. Not being a mathematician, I am enlisting the help of Windows Calculator.

I'm assuming this theory comes from teenagers and men in their early twenties, so let's assume you are 20. Easy enough - minimum age is 17. That seems to work - it's over the age of consent and sensible enough to know that if they are dating a 20-year-old, he/she is three years older and possibly involved in different day-to-day activities, like higher education or even the hideous "work" that people keep talking about. (Unlike the couple I knew who were 14 and 21 - that was just a bit creepy.)
This also works if you're 25 (19 and a half) or even, at a push, 30 (22... although that's a real push). But any older than that and this theory falls to pieces.

Okay, they say all's fair in love and war and, I suppose (although I don't agree - war is not fair, and love sure as hell isn't), as you get older the age difference isn't exactly the same as it makes a difference when you're younger. But seriously - say you are 70. 42 is the age calculated, and that's not young enough to be a toyboy/girl (do toy-girls exist? Or are they called something different?), but old enough to know better, surely? 42 (28) provides more of the result I was thinking of. But, as I say, it's flawed.

And what if you are a child? I mean, I was romantically (although not sexually, that'd also be weird) interested in a young lady when we were at the age of six - and that brings up ten. 6 and 10 kinda doesn't work, when you're young a couple of years makes all the difference.

I'm not really going anywhere with this. It was just something I was thinking about on a train late last night, and wanted to use it to demonstrate two universal truths: people are stupid, and doing maths sucks.