It’s a mix of issues with both incumbency and health episodes that come with old age.
In practice the legislatures do not sufficiently police the health of their members and remove them if they are in poor health and unfit to serve; and four to six years can be quite a long time for health issues to emerge.
It's not the legislature's job to do so. It's the people's job to do so. The House has 2 year terms and the Senate has 6 year terms.
But there are people that are plenty sharp well into their 80s, 90s. Benjamin Franklin was in office until he was 82. You look at someone like Warren Buffett as a more famous example of someone that is plenty sharp well into their 80s.
People age at different rates, and there's really no telling what sort of advances in medicine might improve this and extend life and healthspan by decades.
The problem is that you can be perfectly healthy in year 1 and have severe cognitive decline by year 3, and voters can't really do anything about that.
as a general rule, right now states have no constitutional authority to institute recall elections for federal offices like Congress, so the only way to delegate that kind of authority would be for the current Congress to vote that into existence.
In practice the legislatures do not sufficiently police the health of their members and remove them if they are in poor health and unfit to serve; and four to six years can be quite a long time for health issues to emerge.