BTW, quick count SO it might not be as accurate but of the airplanes in our 720ish mainline fleet about 270 of them are 1992 or older. Basically, in 2020 they'll be near 28-30 years old or older.

Looking at the 270 number, a tad over 245 or 90% are from the domestic fleet:

• 107 are MD88s or 92% of the entire MD-88 fleet.
• 18 are DC9s and yes, like you, I believe they'll still be here.
• 9 are 767-300 domesticateds.
• 76 are B757s of various varieties.
• 35 are the A320s.

I'll be happy once we order 250+ domestic jets in this decade. Anything less and I'm just not really going to see it as anything other than replacements.

Source:
http://www.airfleets.net/flottecie/Delta%20Airlines.htm

And the reason I used the 30 year old benchmark, right now we only have 2.5% or 18 jets in the fleet flying over the age of 30...