Game Boy - 1989
Game Boy Color - 1998
Game Boy Advance - 2001
Nintendo DS - 2004
Nintendo DSi - 2008
Nintendo 3DS - 2011
Let's see, that's 9 years, then 3, then 3, then 4, then 3. By that logic the new system should be out this year! Whoa!
(Yeah, the thing about console generations is that it isn't always about 5 years, it's quite variable. Usually 5-7, sometimes longer. And technically the 6th generation of consoles lasted like 11 cause the PS2 wasn't officially killed for some time)