It is considered an older bill, but it was unknown at the time. Doctor messed up the address when sending the bill so it took a while to find it.
So if this procedure was done in 2017, and paid in 2018 in full, would it be claimable on the next years taxes?
You cannot claim it for your 2017 tax return. You can only claim medical bills for the year you actually paid them, even if they were billed earlier. So the bill you just paid in early 2018 goes on your 2018 tax return when you file it next year.
So if my mother had medical bills in 2017 and pays them in 2018 they can be claimed for 2018 legally?
Yes, the medical expenses can only be claimed for the year they were PAID--no matter when the bills were from.