Those aircraft (at least in the US) that remain registered (an FAA requirement to operate these aircraft) have lots of operating data. See the NALL report (AOPA and others). In general a 100LL 4-cylinder piston aircraft is the workhorse of the GA fleet, used by flight schools and flying clubs. A 1969 Cessna 172 is likely to be a primary trainer (the first aircraft you step in) because the depreciated cost of the airframe and simplicity of the engine/avionics means a flight school can operate it at a "reasonable" cost per hour for the student and not lose their shirts. Ditto for most aircraft made up to about 1995. Go to a flight school and look at the schedule for such an aircraft and you'll probably see appointments noon-to-night because students desperately need hours for their logbooks, and the oldest planes are the cheapest.
Newer aircraft with engines certified for 91-Octane AVGas and such unleaded replacements generally tend to be cost prohibitive to students. Most are owned by owner-operators, and while some are at flight schools, they are rare. The only real change to the market is the use of Jet-A based diesel engines in some of the new Light Sport Aircraft which are expected to take over the trainer market. Unfortunately a change of engine from a 100LL piston model to a diesel is a very expensive transition, complex permitting process, requires the manufacturer to obtain a certificate from the FAA, and causes the owner to throw away a piece of working hardware (the old engine).