The results aren't in the least bit surprising: you're more likely to stick in the NHL if 1) you score a lot; 2) you score a lot when you're younger. If you're over 20 and you're still playing junior, odds are you haven't impressed a pro team in training camp and you're almost out of options.
Each age data point includes players that are three months younger and three months older. There is a small 'blip' in the data around 17.3 years old, roughly corresponding to the cut-off date for the NHL draft. Players who just miss the cut-off date and have to play an extra year in junior end up being slightly less likely to play in the NHL because they've lost a year of development.
If you want to draw a line in the sand, it would be this: a draft-eligible player who scores a point-per-game has a 50/50 chance of playing regularly in the NHL.