The widest range is 1991-2004.
I don't know if it was coincidence that those were the years Squaresoft was at the top of third party developers, but it sure seems like that was why it was the golden age. Yeah Nintendo was at their peak too, but Squaresoft provided the 'other half' for gamers to enjoy the more emotionally poignant side of games. Both companies complimented each other by filling all the spectrums to enjoy games during their (The companies) peak years; and both of their art styles were anime-like or at least colorful while their characters were full of life. Thus, many Square primary fans loved Nintendo games and vice versa.
To narrow it down a little to a more accurate range: 1994/1995(The period where 3D gaming started) - 2002 . The latest year you could argue might be 2004, but then that's it. That's when Japanese developers started to lose ground to Western and European developers.
This was also the beginning I believe of when Japanese developers started to try and imitate Western developers instead of sticking with their own style of games. The homogenized look of gaming began around 2005 when RE4 completely changed the series' design. It was a huge hit and for many their favorite RE and some their favorite game of all time.
Many games were influenced by RE4 and continue to be. In 2004 of course HL2 and San Andreas were huge hits as well. God of War came out along side RE4 blurring the lines of what once were Japanese specific kind of games; God of War and it's combo system if I'm correct was influenced by DMC.
Edit: Oh yeah, can't forget about Halo 2's influence. The most revolutionary console multiplayer game of all time released in 2004 as well. Another huge hit for Western developers.
Lube Me Up
