Are you a retired soldier who is "still fit to fight" Is PT still part of your day? Are you a "mobilization asset", ready to return to active duty if needed?

Retired soldiers are now authorized to purchase and wear the physical fitness uniform (PFU) and the improved physical fitness uniform (IPFU).
It wouldn't be our magnificent Army if this didn't come with the appropriate regulations
See http://www.apd.army.mil/pdffiles/r670_1.pdf for all the details and information on how to properly wear said uniforms (refer to Ch. 30-3)

Wait, so are you not allowed to wear your other uniforms from the army, if your retired?

There are times when you can't and times when you can (parades etc.). It's all covered in the regs..