Akira Toriyama agreed

& he wasn’t even 30 yet.

At the time Son Gohan was turning into the main character, wasn’t he?

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Yeah! I figured Goku was getting old, and it was time to switch over to the next generation.

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There you have it. 30 is death.

Dr. Gero felt the same way.

Chapter 340 (DBZ 146), P8.6-7
Goku: “I see…But did you spy on the battle on Planet Namek too?”
No.20: “There was no need. By the battle with Vegeta and co., we had already completely grasped your power and techniques. We calculated that no matter how much you improved afterwards, considering your age, you wouldn’t have any increases as enormous as you had before…”

You reach your physical peak around 25, anyway.

Scott said:

You reach your physical peak around 25, anyway.

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Maybe. Long distance runners peak early to mid-30s. MMA the peak nowadays is considered to be early 30s.
This also assumes everything else is equal, such as training protocols, nutrition and health. It's more theoretical. In reality, someone's physical peak could be at 15, 25, 40 or whatever, depending on their life.

Scott said:

Dr. Gero felt the same way.

Chapter 340 (DBZ 146), P8.6-7
Goku: “I see…But did you spy on the battle on Planet Namek too?”
No.20: “There was no need. By the battle with Vegeta and co., we had already completely grasped your power and techniques. We calculated that no matter how much you improved afterwards, considering your age, you wouldn’t have any increases as enormous as you had before…”

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I don't think shows the same view at all (that Goku was too old to be a main character or even that Goku was too old to physically improve). He's just saying there won't be as big jumps in strength, because he's no longer a growing child like he during Part 1. It's just the same for, say, a well-trained weightlifter between 20 and 25: they'll get stronger in significant ways (eg perhaps enough to go from regional to international level), but the strength jump won't be what it was between 12 and 17.