Kawhi and Jimmy Butler are a couple rare exceptions. Generally, NBA superstars reach that level before age 23. Go back and look at guys like Durant, Anthony Davis, Kobe, LeBron, etc and you'll see all of them more or less reach their superstar stat output by age 21. That's why you should prepare yourself for Tatum (a true superstar prospect) to reach that level as early as next season.
With Kawhi and JImmy, they had uniquely weird developmental paths where they slowly improved over the course of 5 seasons getting to their "star" level at age 25.
That's not really typical.
So I understand how you can look at someone like Jaylen's current trajectory and say "oh, he's on path to be the next Jimmy Butler in a few more years." To be clear, I was actually on board with the Jaylen Brown pick on day 1 and said I hoped he could be Jimmy Butler in 5 years, so I'm not totally dismissing that concept. But I think we need to be careful about not setting our expectations too high.
Jaylen Brown 2nd year: 14.5 points, 4.9 rebounds, 1.6 assists, 1 steal with 46.5%/39.5%/64.4% shooting.
Jeff Green 2nd year: 16.5 points, 6.7 rebounds, 2 assists, 1 steal with 44.6%/38.9%/78.8% shooting.
If you were to back a decade and ask me about Jeff Green, I would have told you he was on a path to stardom and could probably be the best player on a team if they didn't already have the phenom Durant. Green was a similar player. Decent shot, strong defender, super athletic, explosive, limited offensive skill set.
I remember getting a glimpse into how good Jeff Green could be during Feburary of his 2nd season when he put up averages of 20.5 points, 9.5 rebounds, 2.7 assists with 45.7%/40.5%/79% shooting. That was a whole month of Green looking like an all-star during his 2nd season.
In a lot of ways Tatum/Brown reminded me a TON of Durant/Green this season.
For what it's worth, I think Green was poorly developed. He played out of position early on (before it was trendy) and SEA/OKC didn't have the supporting talent, coaching and environment for him to blossom the same way Brown can. Also, Green always seemed a little checked out. Brown seems motivated and hungry to go beyond Green's ceiling.
But just because he's improved doesn't mean he's a guarantee to continue improving.