Software is available for 2015, 2016 and 2017.  Years prior to that are not available.

Yes, you need to since each year the tax laws change both at the federal and state level. If you try to file your return using only one year of software, the tax calculations will be incorrect and the IRS and/or your state will likely change your return to the correct year, or flat out reject it. 

To get the previous years software you can click here: https://turbotax.intuit.com/personal-taxes/past-years-products/

Just be aware that the deadline to file 2015 for a refund is April 15, 2019.

Be aware that you must PRINT and MAIL your returns for previous years --ONLY the current tax year (2018) can be e-filed.

NOTE:  When mailing a printed return it is always a good idea to go to the Post Office and mail it 'Certified' with 'Return Receipt'.  That way you will have proof that they received it and when it was received.

Also, NEVER mail more than one return in the same envelope, to ensure that they do not get bundled together or otherwise mishandled.