I'm in my 60s, too, and support younger politicians if they're qualified. But many aren't and your example of Bernie proves that age alone should never be the determining factor. Not everyone has Bernie's sharpness of mind at his age. In fact, few do. That doesn't mean all older politicians should be "put out to pasture".
I don't want Biden to run again in 2024 but there's no denying the improvements he's made to the nation, and he learned to get things done in Washington over the course of 4 decades. Younger politicians, as much as I admire any of them, just lack the experience needed to function in Washington politics.
Nancy may be old (IS old), but she has gotten a great deal accomplished in her multiple terms as Speaker of the House. I doubt any younger politician could've done the same.