I'm in dec-1969 in my world. My maintenance costs are: 47,934,883 and my fleet average age is: 2,2 years.
But, these costs are from an 550 aircraft fleet.
I have: 160 Bac 1-11-200(those are the bigest aircraft type in my fleet with the oldest reaching 4.5 years - soon will be retired), 110 DC-9-30, 120- DC-8-62(wich are more profitable than the IL-62), 20 Fokker F-28(i"m using this for my less-demand base, St. Louis and 90 HS-121 Trident 1E-104(a good aircraft, less fuel and price than 727-100 with more seat capacity) and 50 Ilyushin IL-62 .
So, in 1970 i thank the comments about 747, because i'm afraid about their really profit, because they have more than 400 seat, and my biggest aircraft in fleet are the IL-62.