I think there's a few reasonable reasons. I think 65 might be too low, I'd probably go with 70, but one reason is just to make room for new legislators which is

I think there's a few reasonable reasons. I think 65 might be too low, I'd probably go with 70, but one reason is just to make room for new legislators which is good both to get new ideas & perspectives in from time to time, and also to ensure our government isn't excessively skewed towards the perspectives of older people. The average age of a senator is 64.3 years[1]– the average age! Average age of US citizens is 38.8 years. I don't think this needs to be exactly the same & I think it's good for senators to skew older as wisdom accrues with age, but this doesn't simply mean "older is better." Mostly I think it would be good to build the next generation of leaders periodically. Senate seats are and congress are zero-sum, the next person can't get into that seat until someone else gets out, and those "years as a legislator" are important to operate effectively. 1: https://crsreports.congress.gov/product/pdf/R/R46705#:~:text=Senators%2C%2064.3%20years.&text=The%20overwhelming%20majority%2C%2096%25%2C,Congress%20have%20a%20college%20education https://crsreports.congress.gov/product/pdf/R/R46705#:~:text....