Tire age

That inside rubber dust/debris is the inner liner coming apart and as said it it normally caused by running a tire very low or having a flat tire and driving on the tire for a distance. You can usually see the damage on the outside of the tire as a ring around the sidewall of the tire where the rubber is worn down. Any tire with that type of damage should be discarded.
Tires have had DOT numbers since 1971.
I was in the tire business from 1973 -2008.