Would you support a mandatory retirement age of 75 for US House, US Senate & US Supreme Court Justices and if not why?

No.

1. I think that 75 is already too old, especially because they won’t let go of their positions until their terms end even after the “mandated” age of retirement (unless the law specifically forbids taking a position you won’t be able to complete)
2. Politicians will argue that this age is either too young or too old and will either never update this law, or update it so often it becomes meaningless.

An alternative could be to set the limit to a percentage of average life expectancy, or some other variable, so the law isn’t as easy to ignore or mess with, the law can remain unchanged for decades and remain relevant without adverse effects (hopefully), and politicians are encouraged to improve the quality of life.