How do I open prior-year tax forms with the current year TurboTax?

[DoninGA]

You have to sign onto your 2016 online account using the exact same user ID you used to create the prior year accounts and download the PDF of the return from the Timeline

[Critter]

It cannot be done ... the 2016 program only opens 2016, 2015 only opens 2015, etc. 

You should have saved a PDF of the return which can be opened by any PDF reader.