I hear what you're saying because it is true that there are plenty of players beyond 23 years old who are doing decently if not amazingly. This age discussion is something that has always been hit with claims that the fall off happens at X years old with the examples being spotty at best. The turnover for pro players is naturally high in a franchised esport system. Some players will only get one year or less to make themselves known before they drop into Tier 2 obscurity or worse.
I feel the reason why some fans claim that the prime age is so young is because we get very hyped about players who aren't even eligible to play for Tier 1 due to age restrictions and laws. The past few years, you could see it in Sayonara, Lar0k, Loita, Canezerra, Ion, Kozzy and so many more examples. All of these players had some serious hype around them when they were <18 in Tier 2 and the desire to see them shoot heads on a big stage is palpable. Tangent: the age situation is where CS has somewhat of an advantage because we saw donk going nuts when he was 16-17 years old. If he were a Valorant pro, he would be forced to chill in Tier 2.
Hopefully the new format for next year will give some hope to the younger players who wouldn't naturally get a chance in franchising, but to get back to the point, I agree that the prime age and fall off age are wrong - if anything it's more case by case than anything.