Regarding the age that D men mature... I did not save my old study on it and it was pretty simplistic tbh.
So I decided to do another one quickly that is far better.
I took 100 defenseman currently in the NHL, I tried to include as many as the best 100 as I could, but it is probably more like ~85 of the best 100 currently and ~15 who were/are also top 4 guys at some point.
I then split them into 3 groups, which were:
Top tier #1s: This group has 11 Dmen in it. (Weber, Suter, Doughty, Keith, Subban, Chara, Hedman, McDonagh, Ekman-Larsson, Karlsson and Pietrangelo.)
Legitimate #1s: This group has 17 Dmen in it, not all are currently #1 standard but I believe all were at some point or are currently. (Vlasic, Giordano, E. Johnson, Carlson, Faulk, Seabrook, Kronwall, Shattenkirk, Fowler, Letang, Josi, Bouwmeester, Phaneuf, Markov, Gonchar, Hamhuis and Yandle.)
The other 72 dmen comprised guys like: Streit, Coburn, Stralman, Burns, Methot, Brodin, Vatanen, Lindholm, Green, Hjalmarsson, Girardi, Ehrhoff, Edler, Voynov, Staal, Sekera, Leddy, J. Johnson, Muzzin, Greene, Hamilton, Spurgeon, Petry, Wideman, Mitchell, Orpik, Bieksa, Daley, Phillips, Carle etc... that is just a 30 player sample of the 72 but you get the gist. Some of these guys are #2s, some are #4s... but all are or were capable of playing a top 4 role. I picked them mainly on who is currently best in the league, but also factored in minutes they receive and points they get.
Then I looked at two things:
When they first became top 4 Dmen, ie 20+ minutes a game.
When they first became #1s. This is harder to discern, but I looked at Norris voting, guys playing ~24+ minutes and points.
Then I averaged each group and the overall cohort.
The results were:
Top tier #1s:
Became top 4: 20.73 years of age.
Became #1s: 22.55 years of age.
Legitimate #1s:
Became top 4: 21.35 years of age.
Became #1s: 23.41 years of age.
Top 4 Dmen:
Became top 4: 22.74 years of age.
Overall:
Became top 4: 22.28 years of age.
Became #1s: 23.07 years of age.
(Note: The ages I used were the age as hockey reference does, so their age on February 1st of the given season they were 1st a #1 or top 4.)
Worth noting the median age a player became top 4 was age 22.
The median age a player became a #1 was 23.
The mode age a player became top 4 was age 22.
The mode age a player became a #1 was age 23.
The youngest a player became top 4 was age 18. (Ekblad.)
The oldest a player became top 4 was age 31. (Streit, no-one else was over 28, only 20/100 were age 25 or older, with 6/100 age 27 or older.)
The youngest a player became #1 was age 20. (Doughty.)
The oldest a player became #1 was age 28. (Kronwall, him and Giordano (27) are the only 2 age 26 or older. Chara and E. Johnson at age 25 are the only other 2 of the 28 '#1s' who became #1s at 25 or older. 6 more were 24, (Suter, Keith, Carlson, Yandle, Markov and Gonchar) so 10/28 were 24 or older, the other 18 were 23 or younger.