Re: For, or Against... You're both wrong.
As pointed out, removing a justice is the same as removing anyone else. Article 2 Section 4 of the constitution reads "The president, vice president and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors." and as judges are civil officers they can be removed. Further the "good behavior" in Article 3 implies that impeachment is possible for other, presumably lesser, reasons than "treason, bribery, or other high crimes and misdemeanors."
As for the ages of the judges, only half are beyond 67 which is what Social Security considers full retirement age. A number which I feel only proper to point out that there is an excellent chance that it will be higher for our younger commentards. I also don't see anything backing up your assertion that "it's a scientific and medical fact" that age necessarily means declining mental faculties. I would say there is quite a lot of evidence that would support that it is the young whippersnappers who, without any experience, lack any sensible judgement at all and some of said cohort even revel in that fact and promptly post the resulting video to youtube or send the nude selfie to their [b,g]f.
Now consider that Senators must be at least 30 and Representatives at least 25 and you might realize why it seems that they are so old to you. The average age for Reps in the 114th Congress was 57 and the Senate was 61 which is lower than the 63.1 years for the 111th congress a few short years ago. That's not terribly old when you consider that congresscritters typically don't run or get elected until they are near 50. How many 32 year olds are even running for a political office at the national level? Probably not many have the resources or recognition to start a campaign and those youtube vids likely aren't helping.
Oh, those boneheaded laws the congress comes up with, they were bought and paid for by the people who put those critters in office.