Because that would be absurd. The 20-yr gap is an optimistic speculation. Was it based upon technology only? Or how about currently deployed technology? What about technologies that are on the cusp between development and deployment? The USAF's best, the F-22, took over a decade of development. Now comes the time of deployment and with it, research and development of tactics to exploit its best qualities. That is an intangible that at best extremely difficult to quantify and can only be experienced, even as a spectator. So how long will it take the J-20 to complete its development? How long will it take the PLAAF to refine the current air doctrines to best exploits its best qualities? The J-20 is effectively China's first unassisted fighter in development. China has no foundation of/from low radar observable platforms in order to find out how to best exploit its best qualities. The PLAAF will be like US with the F-117. And I could go on and on...
Sorry, but the 30-40 yr gap estimate is more realistic.
This is not the correct way to look at it.
There are 3 primary factors that will decide the performance's of the J-20 and that is stealth, avionics and engine technology.
Stealth - China is now not where the US was with the F-117! The main reason for this is that China has simply looked at the F-22 and managed to save itself decades of experimentation by simply copying major parts of it's design into the J-20. The other fact is that the Chinese now have
massively powerful supercomputers to design optimal stealth shaping that was not around in the 1970s with the F-117.
Avionics - China has already produced AESA radars on both destroyers and AWACs aircraft. It more than likely has produced a miniaturised AESA radar for
the J-10B fighter. The AESA that is currently being developed for the J-20 may well turn out to be better than the one that the F-22 currently has.
Engine - While the WS-15 engine for the J-20 may well lack in longetivity and maybe fuel comsumption compared to what the F-119 has in the F-22,
what really matters as far as fighter performance is concerned is the amount of thrust that it generates at "normal" and reheat thrust. With China
producing 30,000Lb thrust Ws-10 engines by 2010, they should not have too much problems with getting an engine around the 40,000lb mark by the latter
part of this decade.
When the J-20 enters service, then China most probably would be around a decade behind the US in fighter technology.