Doesn't really count, because Page and Jones were experienced musicians at that point, but Plant and Bonham were 19/20 when recording Led Zeppelin I, and Ozzy and Tony were 21 when they recorded their debut.
I'm not arguing that it's not impressive what Metallica did, but I'm not putting Ride The Lightning up there with Led Zeppelin I and Black Sabbath.
EDIT: It's interesting; I've been looking up musicians and their birth dates... It's amazing to me to see where musicians fit into the larger time span. You've got a window of '42 to '45, where McCartney, Lennon, Jagger, Richards, Blackmore, Page, then you have this window from about '48 to '50: Plant, Bonham, Simmons, Iommi, Osbourne, Banks, Rutherford, Collins, Halford... Then you dig deeper, and you see that Page and Jones - in the first group - teamed up with Bonham and Plant - the second group. So did Jon Anderson - the first group and Squire - the second. Meaningless, I guess, but fascinating.