Seventy? They’re not airline pilots! They do wield more power than pilots, but none of them have any power in isolation. If a Justice becomes senile, there are eight others in an unparalleled position to recognize that and do something about it, from gentle encouragement to retire to delaying the court calendar to encouraging Congress to impeach to other remedies I haven’t thought of.
Setting a hard age limit would also further confound the nomination and confirmation process. Having to consider at exactly what point during a future Presidential term a Justice will have to retire adds yet another factor that is not related to suitability for the position, and we’ve got more than enough of those already.
(Seventy really isn’t all that old these days.)
ETA: Not to mention what it would do to Presidential elections! If it was known for sure that the next President would be appointing four Justices, or none at all (barring sudden death), that would have a dramatic and perhaps unwelcome effect on the election. For example, If no Justices were slated to be replaced by the next administration, the fear of Trump might be muted and let anti-HRC Democrats be just comfortable enough to stay home on November 8th. Whereas if there were three or four due to retire, even the most ardent Sandersnistas would have to hold their noses and vote for her.
