It’s partly by design—the word
senate shares its root with the word
senex “old man” (whence also
senile, etc.) because it’s literally a council of elders. That’s what the Roman one was and what the US constitution (among others) was attempting to emulate.
Partly, though, it’s a bit more interesting than that[1]. Wikipedia has an interesting chart[2] titled “Lifespan of each United States president”, which shows that in modern-ish times, the generation from which US presidents come is sticky: you get three presidents born ca. 1855, then one born ca. 1865, two ca. 1875, three from the late 1880s, four(!) from the 1910s (starting with “young” JFK), Carter/Reagan/Bush Sr. oscillate between the 1910s and 1920s, then again four from the 1940s (starting with “young” Clinton) only interrupted by “young” Obama (born 1961). (I could start my recounting earlier and it would still work.) Trump and Biden appear abnormal if you look at their ages, but if you look at their birth years instead they are simply a backslide, after Obama, to the generation of Clinton and Bush Jr.
What does appear true is that generations hold onto their power for longer and longer before the snap transition to the next eligible generation happens. Longer life expectancy due to better medical care seems like a perfectly satisfactory explanation for that.
[1] I first read about this idea a few years ago and I don’t remember where, so I’ll need to attempt my own explanation. Sorry.
[2] https://en.wikipedia.org/wiki/List_of_presidents_of_the_Unit...