Its demographic data; if the apps themselves have any access to it at all, it will be misused by advertising companies to see if they're attracting kids (assuming people are actually truthful; i was totally 18 when i first looked at porn, trust me bro).
Cellular typically uses a separate antenna than wifi, on a smartphone i imagine they're directly on the wireless chip instead of a daughter-board? Laptops and bigger stuff usually routes the antenna wires to the back of the lid, behind the screen, im more used to that
Nah. Rip out the antennae assembly. The phone might be a bit complain-y about it (or fail some kind of pre-boot check and not boot)? never done it to find out though.
Functionally, AFAIK there's nothing really stopping older devices from being updated to use e-sim. They work just fine for emergency service calls, the little chip doesn't matter that much to the underlying hardware
And a young-adult-hood. Crazy, ain't it?