Marketing wise it should be when the MARKET says so. A product by itself, or the company that markets it doesn't decide when a "new generation" starts, it must first take a good hold of the market and make its presence be reckoned with.
I always take the 3DO as an example. It launched at the end of 93. It didn't sell from the start, it failed. Did the 3DO start a new gen? The 16-bit gen was still in its prime, nobody even noticed the 3DO, history can go ahead and lump it with the rest of the 5th gen (Saturn, PSone, N64), but the truth is the 5th generation didn't start until the PlayStation launched successfully *AND* established a solid installed base relative to all previous successful consoles and the whole market, and everything pointed to the consumers (and developing studios as well) moving into it.
So, to answer the OP question : Wii U hasn't started anything as it's been unabled to outsell the current-gen in any period of time consistently. It's the Wii U "next-gen"? Market wise? it'll have to earn its place in the market first, as of now it doesn't have next-gen games (only cross-gen, which don't really count), so it's pretty much a current-gen console until it proves otherwise.
