How do I know whether to use the 1040 form or the 1040 form for seniors?

The 1040 and the 1040-SR are exactly the same forms, just with larger print.

TurboTax will choose the form based on the taxpayer's age.

Let TurboTax pick the one it wants. Your neighbor will be OK.

All Turbo Tax versions have 1040-SR.  You automatically get the 1040SR if you or your spouse is 65 or older.  

The 1040SR is EXACTLY the same as the regular 1040 just bigger type so it prints on 3 pages instead of 2.  And the Standard Deduction is on page 4.  They both use the same instructions and tax calculations.

Form 1040:

https://www.irs.gov/pub/irs-pdf/f1040.pdf

Form 1040SR

https://www.irs.gov/pub/irs-pdf/f1040s.pdf

When you print it will print on the 1040SR even if you see only the 1040 on the screen.  You automatically get the 1040SR if you or your spouse is 65 or older.  In the Desktop program you can tell it which one to print.