Pascal Siakam and Fred VanVleet of the Toronto Raptors are both 25 years old. Typically in the NBA, players enter their prime at around age 25 or 26. Once a player hits 30, their body will slowly start breaking down. Players start to lose some of the athletic advantages they once had.
This is, of course, is different for every player. It depends on position played, minutes played, injury history, and skill-level, among other things.
In any case, Siakam and VanVleet are still very young. As long as no major injuries derail their careers (knock on wood), they each should have at least another five years of prime-level basketball left to play.