This is Jae's age 25 season, so I think he's probably as good as he'll ever be. But he's pretty good right now.
I understand that line of thinking, but I read a study one time that players hit their peak at 27 or even 28.
http://forums.celticsblog.com/index.php?topic=67429.0
Butler didn't break out until last season, his age 25 season. Carroll didn't break out until his age 26 season. There are other examples.
It seems like CBS is constantly pushing players to add to their game by making them more "skilled."
I think he can add some little parts and continue to up his efficiency. Even this year he has improved throughout the year. He is playing the best basketball of his career in December. Why should we assume that is his peak? He continues to trend upwards. That trajectory will even out, but I'm not sure we have any evidence that he is starting to even out.
There was an interesting study done almost 6 years ago now, looking at all NBA players who'd played at least 10 seasons and who played some threshold of minutes. Some people didn't like the gestalt stat used -- I think that's just looking for a point to criticize, and one could use a different stat and come up with something similar. Anyway, that study showed NBA career peaking around age 25, which is where that came from, and what that discussion you linked to was referencing. However, someone did a neat follow-up, which I can't find at the moment, that showed this peak season age had increased over time. As of that time, the peak season for players born in the late 70s had increased to the 27-28 age range. (Being based on players with 10 years of experience after the season ended in 2009, it didn't talk about players born in the 80s, much less 90s as is the case of Crowder.) One is free to hypothesize on the reasons (that author was a fan of modern medicine, whereas I think the year-round conditioning modern NBA players put themselves through is important), but it was an interesting conclusion.
Of course, this peak age stuff is always fraught with the problem that different players are different, but it is likely that Crowder still has a season or two of improvement left, and maybe more. Same is true of Bradley.