Red(s)HawksFan said:
I have to think the "rushing" is related to the amount of time the player is in the system rather than their physical age. Bogaerts started his pro career at a younger age than JBJ for example and had nearly a full season more time in the minors before his call-up. He was arguably more ready at 21 than JBJ was at 24.
I think it's even more specifically about time in the upper minors. Here are the upper minor times, before their first call-up, of the recent young position players:
Middlebrooks: AA 96 G / 397 PA; AAA 40 G / 160 PA
Bradley: AA 61 G / 271 PA (After beginning the 2013 season in Boston, AAA 80 G / 374 PA)
Bogaerts: AA 79 G / 356 PA; AAA 60 G / 256 PA
Vazquez: AA 116 G / 481 PA; AAA 67 G / 274 PA
Betts: AA 54 G / 253 PA; AAA 45 G / 211 PA
With the exception of JBJ in 2013 (if you roll his beginning of the season Boston time into his AAA time), none of them has spent a full season in AAA. And except for Vazquez and Bradley, barely a full season between AA and AAA combined.