Bethel is NOT stress free. It is a most hectic existence, and for those higher up, there is a lot of politics to deal with. I couldn't imagine the level of stress at the GB level.
There are sick people at Bethel, chronic illnesses, depression, cancer, etc, so I don't think being at Bethel necessarily enhances your lifespan. However, that perception could be from the fact that the old timers are able to get comfortably old because they have great health coverage and assisted living facilities (e.g. the "D-Res" at Wallkill) in a perceived spiritually nurturing environment, and there are a lack of younger sick people simply because Bethel doesn't take them or finds a way to get rid of them. If you grow old and are able to go about your work and life productively, you can last a long time at Bethel. But if you start breaking down, Bethel will most likely look to divest itself of you, and send you out into the world, unless you've spent 25+ years there and they wouldn't be able to get rid of you without looking really bad.
[What I say is from experience as a Bethelite, not just off-the-cuff speculation like some comments above.]