You should place the var_dump($verscil_huwelijksdat_tot_zestien_jaar->format('%R%a')); line before the if statement if you want to make sure it is executed :). Does it return an integer, a boolean, or something else? And what exactly?
Well, according to the examples in the manual, DateDiff::format() does not always return an integer. That's why I said: you should check what it is returning, because you're comparing it against an integer, and you cannot really compare a string value against an integer. If you know what I mean :). Maybe you can find another way to find out if this is what is happening, if var_dump() isn't returning anything (which I find weird, 'cause for as far as I know it always returns something and prints it on the screen).
Yes, so that basically means that the returned value is NOT an integer, right? :) So that means that you are comparing a non-integer against an integer, which is why your if() statement isn't working as you want it to. I guess you'll first need to convert the returned value ("+1 day") to an integer, and then throw it into your if() statement.