What about the 8,000 to 10,000 feet is bad? Is it the change of pressure or is it the altitude itself? If the suggestion is the latter, then I disagree because many people around the world live at 6,000 feet and above including into the teens in some places. Often times on flights I fly the cabin doesn't go above 6,000.
Recirculated air, I don't know. About half of the air on the plane I fly is fresh air and that is with the recirc fans on.
As far as aging, I think it depends on the type of flying you do. Some people can handle international and some can't. I've flown with some guys who used to work at places like Kalitta and they just did not like the constant time zone changing. Around the world in ten days sounds fun but if the schedule isn't convenient it could be rough. The only thing that I think would age me a little is the early wakeups. I'm still in my 20s so I'm mostly a night owl type. I have read that this changes as one gets older, to where they are more likely to go to sleep a bit earlier and wake up earlier than they did when they were young (and also need less sleep per night). But that is it for me. The occasional 16 hour workday is totally unacceptable (and it seems as though they'll be illegal a year from now) but honestly I am way more tired after waking up at 5AM and working 8 hours than waking up at 10AM and working until past midnight.