30 is different for different players.
Paul Pierce, KG, Ray Allen, etc were all still awesome at 30. Granger missed over a year with a degenerative knee condition, jumper's knee, that doesn't get better really. Since coming back he's been basically a three point shooter, who can't shoot.
42% of his shots have been 3s, he's only making 33% of them. His 2PT% is an awful 38%. He's still rebounding and defending okay with his length, but offensively he looks very cooked.