I agree with the concept "age limit".
Make it around 72-74 or so for federal judges and Supreme Court justices.
This MUST be in the form of a Constitutional amendment so that it cannot be challenged (in court, of course) as being "age discrimination". It will be "Constitutional" by virtue of the fact and reality that it's a Constitutional amendment, and that will be that.
Having said that, in order to assure its passage, include a clause that EXEMPTS ALL SITTING judges and justices, for a period of twenty years. That will permit many (if not most) judges/justices to serve [almost] as long as they wish.
We need a similar age limit for Congress and the presidency as well.
Make it, say, 70 for Congress (no one having attained that age will be eligible for election or re-election).
For the presidency, I would go even a bit lower. 67 ought to be "the cutoff" to be eligible for the first term.
Again, exempt all sitting officeholders for a period of twenty years.
Term limits will never, NEVER, EVER pass muster with Congress.
If we wish to "limit them", it must be through other vectors.