Do all years fit?
 

I have a '13 BRZ, curious if the exhaust from header back off a 17 or 18 will fit?

only header/overpipe/front pipe will fit.

Where it join between midpipe and muffler is different.

So the entire exhaust from a 17+ won't fit or I can't mix and match due to the differences?

you just cant fit axlebacks of the different generations

Can't mix and match between years for the midpipe and axleback. If you get something that has both parts (i.e. catback) as so many do, you're fine.

ok, i am just looking to replace the OEM from the header back with the newest 'used' OE i can find. All I need to know is if 17+ will fit or if I have to just stick with -16

They told you already.
Catback/Header yes.
Midpipe/Axle back no.

Hmm. Is there any noticeable difference how MY2012-2016 vs MY2017 OEM catbacks sound? Yes, if changing all the parts including midpipe, it will work, but why change one thing to (almost) same (but fitment at one joint)?

Mine is getting rusty, and I will be mounting a header and the car will not see another winter (it has seen a little winter to this point)

You sure about that? looks like i cannot mix and match but i can take the entire thing...

You cannot mix & match if you use those two bits that have different joint in 2017' from different year cars.

Header-any/Overpipe-any/and then choose combo, old Midpipe+old axleback(mufflers & tips part) OR new midpipe+new axleback.

But as you specified, that you want to change "headerback", that is WHOLE exhaust from cylinders to tips, including header-overpipe-midpipe-axleback, there should be no issues.

P.S. Rust aside, i'd do something only when it's rusted through and started leaking. Noone looks under car to see if there is or isn't rust. Rusting exhaust bits, especially in place with "salty" winters after few years is norm.