Confusion RE "Youngest Retirement Age"

[steven_glass]

I am a retired federal law enforcement officer and retired on disability in 2015 (age 59.5 years). My mandatory (20 year, 6(c)) retirement date would have been in April 2018 (age 63 years) thus I was three years short of full retirement. I have taken the standard deduction since retiring.

Q: What would my “Youngest Retirement Age” be?
Q: Where in Turbo Tax would I go to modify this amount?
Q; Why does “Youngest Retirement Age” matter when filing a Federal Tax Return (a 1040SR)?

Thank you for your time and assistance in this matter – it is greatly appreciated!