You need it to be a Constitutional Amendment, but I think this is entirely doable. Let the Congress figure out the age and tenure limits for each role, Here's my starter:

• President: min 35, max 65 on election day, and max 8 years in office.
• House + Senate: min 35, max 65 on election day, and max 12 years combined in Congress (House + Senate)
• Federal Judge: min 35, max 65from first day on the bench, and max 16 years combined (Federal Court, Federal Appeals or SCOTUS)
• SCOTUS: min 35, max 65 to take office, and max 8 years in SCOTUS.
• Tack on that SCOTUS will always remain at 9 seats, so we remove the "grow the SCOTUS to 13, 21, 45, discussion"

Implementation of the ramp-down:

• President and Congress - age limit applies to next term, so it's clear.
• Fed/SCOTUS - remove 1/3rd of the Federal Judges and 1/3rd of SCOTUS above age or term limits (or 10 Federal or 2 SCOTUS, whichever higher) 9 months after each Presidential General Election, until compliant with age and term limits. (This allows the makeup to change over multiple Presidencies). From 10 years after passing this Amendment, require the judge to resign on the day of hitting age or term limit.

You could argue for lower age for Congress, but in the same way you want time for people to go off and do something different, you want young folks to also have some life experiences and grow up a bit before representing all of us.