Look at the pay for members of the House and Senate, in real dollars, over the last 50 years. Also pay attention to how much stupid noise there is about how members of Congress are supposedly overpaid. The pay for all US Senators combined (under $18M) is less than half of what LeBron James makes (over $41M) in salary alone in a year.
> They exist (if you define "young" as under 40)
In political terms, at the (European) national level, "young" is typically under 50, and "old" is over 70. Acquiring reputation and solid power base takes time.
Looking at the UK: Tony Blair was considered very young when he became PM at 44; Thatcher was 53, Major 57, Brown 56, and most of their predecessors were much older. Cameron was 43 but again May was 59 and Johnson 55. Backbenchers will typically enter Parliament around 35-40.
In Italy you can basically add 10 to all those numbers; the current PM (or PdCM, for the purists) is 73.
Looking into the US senate, I fail to see that trend. In the last presidential election, both candidates were older than my grandparents.
Even in my country, seeing a really young person in a political position is very rare. They exist (if you define "young" as under 40), but they are rare. I don't think age worries are a factor at all.