Prepare yourself for blowback on this one. A lot of people think being a young team doesn't matter. They're wrong.
Statistically speaking, the last time our defense was this good was in 2003 when we were 2nd in the league against the run and 1st in the league against the pass. But that might have had something to do with teams not needing to score many points on us, since our offense that year was 21st in the league in points!
So the real answer is that the last time we had a defense this good it was 1995. That defense from 1992 to 1995 was really, really good.