Average lifespan of a business jet?

There's a rumor floating around the office that my company is
considering purchase of a used bizjet. They have a reputation for
trying to do things on the cheap and seem to be partial to Citations
because we're located in a town that has a Cessna repair facility.
I've seen multiple listings for such planes varying from 15-20 years
and older with TT ranging from as few as 3700 up to 10000-12000. So I
wonder...how old (how many hours) does such a plane get before it has
(generally speaking) outlived it's usefulness? Depending on what they
decide to buy, I'll have to determine whether or not I want to fly in
it.

GrtArtistea non-pilot, not wanting to end up like Payne Stewart