Booglebox -
Reportedly, the avg number of cycles for DL's MD90 fleet is only in the 20k ballpark figure. (Some 2nd hand frames as low as 15k) The MD90 is approved for 60k cycles. That would suggest that at 13-18 years old, the fleet is 1/3 through their useful lives (give or take). Potentially another 20 years.
The MD88 could have 10+ years of useful service life; assuming DL decide to keep it operating that long.