I'd assume that target dummies are usually airframes that are either outdated (an A or B model when we're at D, E, F, G etc) or past their useful (human operated) life, i.e. too many hours, damaged, cracks are forming, etc.
QF-16s are only certified for 300 hours in the air(after conversion) before they are blown up or time expired. An F-16C flies 400 hours between phase inspections in a fighter squadron with more than 3000 hours total airframe time.