Sampras was never the youngest slam champion. Chang is younger than him.
So Chang was the first of his generation to break through at the French, and was still the youngest by about 6 months then, 23 and a half when Kafelnikov broke through in 1996.
Anyways, seeds of the breakthrough players:
Borg (3), - 1974 defeated Ramirez (9), Solomon, Orantes (14)
Wilander (unseeded) - 1982, defeated Lendl (2) 5 sets Gerulaitis (5), Clerc (4), Vilas (3). Helluva a draw. He beat everybody!
Connors at (1) was out in the Quarterfinals. Vilas was the last of the old guard and lost in the finals.
Chang (15) - 1989. Sampras (unseeded) (2nd Round), Lendl (1)!, Edberg (3), also five sets. Wilander defeated by Chesnokov. Becker defeated by Edberg. Agassi defeated by Courier, Courier by Chesnokov.
Chesnokov defeated Courier, Wilander but fell to Chang. Edberg defeated Becker, and Ivanisevic! Interesting draw there...
1996 French Open:
Beat Krajicek (13), Sampras (1) (his only clay Semis!), Stich(15),
Sampras beat Bruguera, Courier, only to fall to Kafelnikov. Ferreira beat Kuerten, beat by Courier, Agassi was a non factor carving out a QF for Krajicek. Ivanisevic had a cheap draw.
Really tough draw for Chang (4). Chang beat Edberg again, only to lose to Rosset. Goran never faced a seed and only faced Rosset in the QFs. Rios nearly made the QFs, losing to Pioline, but beating Korda.
Stich opened up his path through Muster. So lots of seeds fell quite early, and all the servers did well, Krajicek, Sampras, Ivanisevic.
And we all know what happened in 2005, with Fed in the Semifinals.
So there we go. Only once has it been an unseeded youngun, with unseeded Wilander.
Only once has the draw been wide open - in 1974. Every other time, the old guard was in the finals, or the semifinals as the case was for Federer/Nadal. Wilander/Vilas Stich/Kafelnikov and Chang/Edberg.