Jim
It would be interesting to know what angle your research is coming from in a little more detail. Whatever information you get for the 744 is likely to not be statistically relevant. The sample group of pilots and airlines for the 744 is simply too small (probably a few thousand pilots globally compared to several tens of thousands each for the 737 and 320s).
With 651 aircraft in service between 66 operators, that is just under 10 aircraft per operator. Hardly sfficient to base any meaningful statistical analysis with regard to upgrade potential in the light of any particular economy, and local influences will cause too much fluctuation between airlines and regions to not be taken into account.
Are yo sure you have though this through from a research point of view? The 747 may be the one that the public know and love, but if you base any statistics on this aircraft alone without taking the huge number of variables into account, you will have invalid data and therefore you will have a misleading (and inevitably "spun") article.
I know of a couple of guys who made jet skipper before reaching mid 20s, but all on narrowbody. Until recently 744s were mostly the domain of the legacy carriers most of whom operate (or operated) a strict seniority list as pointed out by Joe.
RIX