In most ways you compare current consoles to each other, and determine which of those current consoles are the best during any period of time.
In this case the three primary consoles available right now are the Wii U, PS3 and Xbox 360. They are all fairly close in power and can play the same games pretty much. 3rd party games can be played on all 3 systems, so I would say they are all part of the same generation.
Now when the Ps4 and Xbox Bone come out....things will change again. I would not put the Wii U in the same generation as them if it can not run the same games (3rd party games). In theory at least, it won't have the power to run the newer games so...you might as well call it part of the 7th generation basically.
In reality...it will be part of the 8th generation of games....but not until another generation actually starts. It doesn't push the envelope of power enough to start a new generation on it's own....it just pretty much catches up to the rest. That being said, once a new generation starts, the Wii U will still be in contention and will still get people to buy both systems and new games...so it will still be a viable system in the 8th generation.
It just can't start a new generation itself because it's not capable of running games that other systems (from the 7th gen) can not run. If it's not better then other systems in the current generation, it certainly can't start a new one.
So that is why the 8th generation hasn't started. It will start in november when the new systems start being sold, and I guess specifically once a game is sold that can not currently be run on current gen systems (which considering so many games are being made/sold for both Ps3 and PS4 etc...that may be awhile longer!!).