How do I open a .tax file from early tax years

You need that year's tax program installed.  

After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to File-New Tax Return).

Then go to FILE -Open and find your return.

If you have old .tax files you can try converting them to pdf files at  Www.Taxprinter.Com 

 It might be expensive.  They are not connected with Turbo Tax.  Use it at your own risk.

Not sure what you mean by "early tax years."   Are you referring to tax files stored on your own hard drive?   You need the software for the tax year to open a tax file.  Or did you save it as a pdf so that you can open it more easily?

https://ttlc.intuit.com/questions/1901659-find-your-tax-data-file-in-mac

https://ttlc.intuit.com/questions/1900721-find-your-tax-data-file-tax-file-in-windows