It is true..age is a factor. The CASA 101 (the jet) is circa 1980s and the Chilian T35 (the piston) is a late 70s design. The jet is to be replaced in the SAF. But no reported replacement for the T35 thus far.
But many old designs have long life. A key factor is role and utility. A basic trainer is just that...basic...it need only be strong (to withstand the less than gentle handling of student pilots) and reasonable through life costs. The De Havilland Chipmunk is a classic example of age and durability...still lots of life left in the airframe...hundred's still flying, but its military training role "death" was more about the old engine design and need for a more "advanced" cockpit to get pilots ready and into Tucano, Hawk T2 and 4th and 5th generation fighters.
Military servicing regimes are generally intensive and therefore more expensive...outsourcing to civilian contract is now perceived as the "cheaper" option. But sadly not the most expedient in some cases....it takes nearly 3 times longer to get a UK military pilot through the training system than it did 2 or 3 decades ago.
You are right though...budgets...if you want the facts...always follow the money..