I think it was for a metal washer/shim in the rear suspension that moved something by 1/64 of an inch out of Nascar specs. I seriously doubt it gave him much of an advantage if at all.
And the "Youth Movement" I think has more to do with money than anything else, Matt Kenseth, Kasey Kahne etc... can command a big chunk of change for their contracts. Now Eric Jones & William Byron will probably take 1/2 or less of what the guys they are replacing for salary. Also Nascar tv ratings have tanked from 10 years ago. The 2006 Daytona 500 had 37 million viewers, the 2016 Daytona 500 less than a 1/3rd of the 2006 Dayona 500 viewers tuned in, just 11.4 million. The lowest rated 2016 races just had 1.5 million viewers while 2006's lowest viewed race had 5.6 million viewers. Sponsors aren't throwing the cash at teams that they used to or completely backing out of Nascar period. Look at the title sponsor, Monster only paid 20 million to be the title sponsor and they haven't renewed their contract for Nascar or SHR #41 so Kurt could be out of a ride at the end of the season as well.
If Brian France and his cohorts would stop screwing around with the racing maybe the ratings would plateau or slightly increase instead of continuously dropping year to year.