Mr. Barron, in the past there have been a few threads about 'the youngest B747 skipper' and nobody seems to have an authoritative answer as most info is always based on rumours or 'a friend of a friend told me' and so the theme resurfaces from time to time, but by reading all posts it seems to me that around 27 - 29 years old would be the youngest.
http://www.pprune.org/south-asia-far...too-young.html
http://www.pprune.org/questions/2566...7-captain.html
I joined my current airline as Direct Entry Captain on B747-400ERF at age 35, I'm currently 37 (second youngest Captain in my outfit joined as DEC at 42) and whilst certainly not the youngest in the industry, it's fair to say that it isn't very common to see many B744 skippers below age 40, due mostly, in my opinion, to 'seniority' issues (especially at big companies).
I think you will find that young B747 Captains were exceptional cases that happened perhaps due to a great demand at the time (e.g. Start-up company, small company, rapid expansion/growth) and they proved to have -despite their young age- the flying and personal skills and ability required to be in command of a B747, but with legacy carriers -unless you join at a very young age- it will normally take any pilot a long time to climb through the ranks and fleets and eventually make it to B747 LHS below the age of 40.
Kind regards and best of luck with your research. =)