Safin won 8 titles by 21, including 2 Masters and a Slam
Hewitt won his 13th title the week after he turned 21, had 0 masters but a Slam and YEC. Won his first Masters the following month.
JCF won his 2nd title a few weeks after turning 21.
Roddick turned 21 during his 11th title (his slam, won a few days afterwards). This capped a rare NA sweep.
Zverev won 6 titles by 21 (April), including 2 Masters.
Still, only 1 of his last 10 finals has been at 250 level, so whilst he might struggle with the 2 week BO5 format for now, his 500/1000 record is decent. 10 titles whilst still 21 is enviable pace.
Yep he was 21 and 11 months, which is what Zverev will be in March 2019. I'm not saying this guy will be anything like the big 3 though as that would be stupidity, just saying he has done a hell of a lot more than any other young player currently on the tour.