The vast majority of the Democratic's higher leadership are elderly. I like Bernie, but it is clear that Republicans, despite being shit, actually have representation of their younger cohort and benefit from that vitality. Just look at the Democrats: inaction and timidity is the default state among their politicians. An old person cannot fully grasp the issues of the day and have less energy to carry out an agenda, whatever that may be. Biden wasn't willing to use the immunity that the Supreme Court handed him, despite fully knowing that Trump won't hesitate to abuse it.
Also, term limits would help prevent the likes of Nancy Pelosi from freezing out potentially great leaders from ascending into higher ranks of leadership - which reinforces the priorities of establishment, rather than voters. We have many mayors and governors among the Republicans who don't have town halls, or simply ignore their constitutes. Sometimes, an incumbent simply stays in office due to no viable competition appearing, no matter the (de)merits of the sitting official.
As to evidence, just lots of history books. It typically isn't the older rulers who change the fate of nations in a helpful way. Their idleness, is typically the cause of famine, decay, and disaster, because they simply didn't care or couldn't understand.
Also, I have an elderly parent, and it ain't no picnic. It is hard to 'adult', when the home owner doesn't want the world around them to change. Opportunities or risk mitigation, neither can happen if the old cannot grasp the implications of inaction - which they are prone to do. It isn't hard to map such biological reality onto politics, because politicians are fleshy humans, with all the issues that comes with aging.
I will grow old, and so shall you - we both will become infirm and dimwitted, and so will politicians we have elected. That is why it is important for the young to elect the young.