Retired need to be 62+(Senior Pass) -- https://store.usgs.gov/pass/senior.html
or have a disability (Access Pass)..
Who qualifies for the Access Pass?
The Access Pass may be issued to U.S. citizens or permanent residents of any age that have been medically determined to have a permanent disability that severely limits one or more major life activities.
A permanent disability is a permanent physical, mental, or sensory impairment that substantially limits one or more major life activities, such as caring for oneself, performing manual tasks, walking, seeing, hearing, speaking, breathing, learning, and working.
If you have ANY disability rating from the VA -- you qualify for the Access Pass...
More info at -- https://store.usgs.gov/pass/access.html