Hedman had 23 points at age 20.
OEL had 32 points at age 20.
Subban wasn't in the NHL at age 20.
Keith wasn't in the NHL at age 20.
McDonagh wasn't in the NHL at age 20.
Suter had 16 points at age 20.
Seabrook had 32 points at age 20.
Green had 12 points at age 20.
Shattenkirk wasn't in the NHL at age 20.
Carlson wasn't in the NHL at age 20 (6 points in 22 games, mostly in AHL that year)
Hamilton had 25 points at age 20.
Yandle had 12 points at age 20 (only 43 games).
etc
Doughty, Karlsson, Ekblad, etc are VERY much the exception. Defencemen, even those who go on to become 40-50+ point guys, typically aren't in the NHL, or put up somewhere between 15-30 points at Rielly's age.