Stevens is 88, I think the oldest ever was Oliver Wendell Holmes who retired at age 91. But in terms of sheer "service" on the court, I bet Stevens has been a justice longer than Holmes.
You're right that a nationally election U.S. Supreme Court would likely cause them to spend obscene amounts of money though. I don't think they should be elected "at large" across the entire country like the President, but certainly they'd be conducting campaigns in several states at once. (the main flaw here is some unqualified zillionaire Oberweis/Huffington type in the legal community could win the primary and then lose to a RAT in the general)
I believe, interesting enough, that the 2004 campaign we had in my state for Illinois Supreme Court, 5th District, was the most expensive campaign for a state office in Illinois history, moreso than any Governor's race. But in the end, what matters is the good guy won, and if Blago had appointed someome to our court instead, he simply would have tossed a socialist sock-puppet into that seat for life.
I hadn’t recalled there being an older Justice than Stevens, but you’re correct that Holmes was even older (he retired 2 months before his 91st birthday, so Stevens still has 2 years and 9 months more to go). But as with both, neither should still be sitting on the court at that age. Also, Stevens has surpassed Holmes’s service (Holmes served 29 years and 1 month; Stevens now is at 32 years and 6 months). Stevens’ predecessor was the longest serving in the history of the court (William Douglas at 36 years, 7 months), but he retired at the more reasonable age of 77 (he was only 40 when FDR named him to the court).
BTW, if you get a chance to read Douglas’s Wikipedia entry would be astonished to see how much Jerry Ford was down on him, yet Stevens has been nothing but a clone of Douglas’s ideology, minus some of the peccadilloes.
http://en.wikipedia.org/wiki/William_O._Douglas