Probably it was a random player of one of those editions before the Open Era where the tennis players taking part in the tournament were mugs from local villages close to the tournament's place and friends of the tournament owners.
About who made it to the R16 at the youngest age in the Open Era, I don't have any clue mate.
Coric is what? 18 years 6 months old?
Safin was 18 yrs and 4 mos when he reached 4th round there in 1998 playing his first ever slam main draw. And he beat Agassi and defending champion Kuerten along the way.
And as already been mentioned there is Chang and many others.