At the start of the 2014 NFL season, the St. Louis Rams had the youngest team in the league. In fact, the Rams were recorded as the youngest team each of the past three years. No official stats have been released heading into 2015, but the Rams are likely still the youngest team in the league.
As far as the defense goes, this same unit has been together for a while now, which is something you want. Regardless of age, the ability to gel as a unit is more important than anything.
The Rams still hold nine players on defense from 2012 including Robert Quinn, Chris Long, James Laurinaitis, Janoris Jenkins and Jo-Lonn Dunbar.