I think the idea is that players tend to hit their physical peak in the 25 to 27 range with their actual game peaking between 28 and 30. I'm not sure how they do it but I suspect one of the reasons they usually place 25 or so as the peak year is because so many players are out of the NBA by the time they're 28 so it skews the studies. But if you're talking about star players the peak tends to be higher. For example, the average age for players on the All NBA teams is 28.
I also think the over 28 or over 30 decline is less precipitous than many suggest. The averages probably work out but I think it's important to separate out the players that have escaped major injuries. If you have a strong player that's been mostly healthy for his career, I wouldn't be surprised to see them raising their overall game even into their early 30's.