TurboTax Online only supports the current 2017 tax year and the prior 3 years (2016, 2015, and 2014).   The latter 3 have to be prepared in desktop software (CD/download). Each tax year requires different year software; i.e., 2015 can only prepare 2015 return; 2014 software can only prepare 2014 return, etc. Prior-year returns cannot be efiled; they must be printed and mailed.

NOTE:  If you prepare returns for multiple years, do NOT mail them in the same envelope. Mail them separately.

You can see prices for those 3 prior years at the link below if ordered directly from TurboTax, but you may also be able to find them at a discount at reputable retailers like Amazon:
https://turbotax.intuit.com/personal-taxes/past-years-products.jsp