At the time The Founders put the Constitution together, people didn't live quite as long (on average) as they do today.
I'll reckon that when someone aged 45 was given a "lifetime" appointment as a federal judge or Supreme Court Justice in 1800, he was expected to serve twenty years, maybe twenty five.
But not into his eighties or nineties.
Of course, the only way to correct this is by Constitutional Amendment.
Perhaps a Supreme Court appointment could be designated a fixed term of 25 years, with the president having the option to re-nominate a sitting Justice with a simple yes/no vote in the Senate to confirm. Upon confirmation, the Justice's term would be extended for a period of 10 years, or until he/she retires.
And also the amendment should state that any sitting federal judge or Supreme Court Justice will retire upon attaining the age of 75 years.