Thanks for the detailed reply. Some of that does somewhat trouble me though.
Firstly, I wonder if you're making too many assumptions based on your own experience of aging. I'm also in my 50s and my experience is much more positive. I'm not aware of any mental decline. I work as a computer programmer and, so far as I can tell, my skills are still improving. Further, a year or so ago I started out of interest re-learning the physics I studied at University, and I've been surprised and pleased to discover that concepts I had a lot of trouble with when I was younger I now find much easier to grasp. I also believe my attitude to life is better: When I was younger, I was pretty unhappy, and generally very angry about things I didn't agree with. Now I'm much more aware that the World is pretty complicated, that there are usually two sides to any disagreement, and that many of the political solutions I (somewhat idealistically) believed in when I was younger wouldn't actually work in practice. I also believe I'm much more relaxed and more ready to consider other points of view than I used to be. Now obviously, I'm not standing for US President or for any similar position, but if I was, I'm pretty sure the 'me' of today would be a much more suitable (or at least, less unsuitable) candidate than the 'me' of 20 years ago would've been. I also don't see any reason to believe that the changes I've experienced are particularly unique, although I suspect they have a lot to do with that I've spent most of my life actively trying to learn and improve myself - something that obviously isn't true of everyone. I also now look after my health and do a lot more exercise than I normally did when I was younger. So it would seem to me pretty strange if, in a very few years' time, I was banned from that kind of position on the basis that I'm somehow mentally unsuitable because of my age.
Granted, it's well documented that most people eventually decline, but as I understand it, that often tends to be a slowing down of some mental processes - it's not a loss of wisdom or intelligence (other than, obviously, if you suffer from particular illnesses such as dementia).
Also, even if there is a decline with age, you have to remember that different people are starting from very different levels of suitability. Compare, say (A) a person with a University degree who spends his/her life contributing to community and seeking to understand society, with (B) someone who spends almost all of his/her spare time getting drunk, watching tv soaps, and who never bothered to get much of an education. Are you seriously going to suggest that person A at 65 or 70 is less suitable to be a leader than person B at 25 or 30? Even if A had declined a bit between 30 and 70, s/he is probably still miles ahead of B in ability and suitability, so it would seem perverse to allow B to stand but not A.
Finally, you mention wanting to understand the direction the younger generation want to take. While that is important, surely you're not suggesting that a leader should be exclusively going where the younger generation wants to go, while ignoring/having no awareness of, what older people are likely to want? Wouldn't a good leader be concerned to represent people of all ages?