FWIW, phenoms usually reach superstar level before the age of 23. So it's not totally crazy to see guys performing at a high level at age 20/21.
LeBron was already averaging 27 points, 7.2 assists, 7.4 rebounds, 2.2 steals with 47%/35%/75% shooting as a 20 year old 2nd year player.
Durant was already up to 25 points, 6.5 rebounds, 2.8 assists, 1.3 steals with 48%/42%/86% shooting his second year at age 20. Up to 30ppg the year after.
Anthony Davis was already up to 24.4 points, 10.2 rebounds, 2.2 assists, 2.9 blocks and 1.5 steals at age 21.
Karl Towns was up to 25.1 points, 12.3 rebounds, 2.7 assists at age 21.
Kyrie was already averaging 22.5 points, 6 assists, 3.7 rebounds, 1.5 steals with 45%/39%/86% shooting at age 20.
Kobe took a while to get going out of high school. His first two seasons weren't great, but he came in at age 18. By age 21, he was up to 22.5ppg. By age 22 he was upt o 28.5ppg.
Tim Duncan was a phenom right out of the gate as a 21 year old rookie.
Shaq was dominant as a 20 year old rookie.
On and on...
So yeah, there seems to be some misconception that you draft a budding young star they will develop slowly over the course of many years and peak at age 26/27. That's really not the case. Sure there are rare exceptions like Jimmy Butler, but generally if you're drafting a phenom like Tatum, there is probably going to be a major leap between year 1 and 2, but all of his main development should happen before the age of 23. He'll pick up stuff as he gets older, but if the guy is going to be a 28ppg player, it's going to happen rapidly in the next couple years.
So in that sense, it's not all that shocking to see two guys picked in the Top 3 performing at a high level. Having extremely high draft picks is essential. Obviously, this Celtic team is special for a number of reasons. Hard to say there is any precedence for what Boston is accomplishing when it's kind of unheard of to have a team threaten for the Finals without their two best players. But you shouldn't be too surprised if Brown and Tatum are playing at all-star levels by next season (especially Tatum who is a superior prospect with a higher ceiling).