From Upfront Magazine (Bar... no cussing):

According to Boeing, there were 1,192 MD-80s
delivered. Of those delivered, 706 are currently
flying. Boeing also states, “70% the MD-80 fleet
(700 airplanes) have less than 35,000 cycles”. Not
convinced that it’s a young pup yet? What if I told
you the life of the fuselage is expected to be
110,000 flight cycles and 150,000 flight hours.
That’s right – Boeing refers to this life expectancy as
the Limit of Validity. That means that most MD-80s
could have 20 – 30 years of service life remaining.
Of course Delta will be faced with making the
determination on where to draw the line. Economic
viability will depend on fuel cost, navigation system
limitations, and inevitable mandates (Required
Navigational Performance, Noise Regulations,
Aging Airplane Safety, etc.). For now it appears that
the MD-88 has a solid spot in Delta’s long-term
fleet plan. And why wouldn’t it; the Mad Dog is only
19 years young.