The oligarchs keep these older politicians around because they tend to serve them well, but it’s not really about age. It’s about the corruption.
Idk man, I agree corruption is the core issue, but from Dianne Feinstein to Joe Biden to Mitch McConnel to probably Donald Trump, I think it's almost the opposite end of the "boy king" phenomenon. Boy kings could be controlled by their advisors because they were immature and unexperienced. Senile politicians can be controlled by their advisors (who are controlled by moneyed interests) because they aren't fully lucid.