Two reasons. First, it's just good policy. You want your representative government to remain representative, especially to those who support the existence of the government, ie those who work and pay taxes. The ideas and the facts of every day life are very different for a 45 year old and a 75 year old. "What about the 25 year old?" you might ask. It is far more likely for a 45 or 55 year old to interact on a regular basis, socially, and at the work place, than it is for a 75/year old to do so. You want people who either live or know people that live the everyday circumstances that affect us all.
Second. The brain shrinkage goes into high gear after the age of 60. Even if you are not obviously cognitively impaired, you are also not at your best, and you never will.
This is a quick search result but there are a lot of other sources. It is well documented.
This next thing is my life experience, and not in any way meant as proof. My father is a well known and formerly brilliant legal mind. He's argued im front of the Supreme Court multiple times. He has beenan athlete since well before I was born and even though he was also born in the 1940's, he still runs five miles every day. Every, fucking day. He does not have dementia. He does not have Alzheimer's. He will likely outlive me. And yet, I noticed some years ago that he is not sharp. The decline has been especially pronounced in the past decade. Is he still smarter than many? Sure. But he is not the man (had he been in office) that I voted for 20 years ago. He is also, as much as he tries to stay informed with current events, painfully out of touch with the needs of anyone younger than 60 (including me).