Re: Old drywall tape
There is no "expiration date" on drywall, it's either done right and lasts a lifetime or it's not and starts failing at some point. What you describe sounds like poor quality workmanship and possibly the wrong materials.
The drywall shouldn't be sagging. Barring moisture problems, sagging can be caused by a number of things, but it's usually either the wrong type of drywall (should be 5/8" or reinforced 1/2" ), uneven ceiling joists, or insufficient fasteners. The direction it is running on the joists has little to do with it, though perpendicular to the joists is most common.
Barring moisture problems, joint tape releasing is the result of either not enough joint compound being applied under the tape or too much being removed during the installation process.
If the overall condition of the drywall is decent, more fasteners can be added and the joints retaped.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!