Tell that to the Penguins - when they won their two Cups:
Maatta came up at 19, 3rd year in 2015-16, and his career flatlined
Dumoulin, 24 year old rookie, 3yr college, 188 AHL games, 38 PO games, played in 2014-15 playoffs before starting in 2015-16
Kuhnhackl, 24 year old rookie with 154 AHl games
Bryan Rust, 23 year old rookie, 4yr college, AHL season, 16 AHL games before called up in 2015-16
Conor Sheary, 23 year old rookie, 4 yr college, 1 year AHL (2 AHL playoffs), then 30 AHL games before called up in 2015-16
Jake Guentzel, 22 year old rookie, 3 yr college, followed by 11 AHL, 10 AHL PO, next year back to AHL for 33 games, before called up in 2016-17
Note that Pens didn't rush Guentzel up out of college at 21 in 2015-16, even though he had a great junior season and AHL time.
When these players came up, they were NHL ready and contributed to two SC winners.