Conversation with a friend of mine after my girlfriend and I had been going out for nearly a year:
"How old is she?"
"How old did you think she was?"
I wouldn't really describe her as having a "loli build" either.
I'd tend to say the word is dual definition - it can refer to a girl with either a young age or waifish build. I do agree that I could never consider Yoko a loli. Sengoku Nadeko from Bakemonogatari is a lot harder - she's 14, so right at the borderline, and she's not really waifish. But its difficult to perceive her as anything but a shy young girl and the loli moniker seems apt for her (DFC not so much. Small yes, flat no.).
I'm pretty much on the same viewpoint on classifying lolis as you are. I most certainly wouldn't consider a 17 year old a loli, no matter how young they look. I dated one earlier this year that looked fairly young so I'd be in deep trouble if those were the standards!
But I suppose the deciding factor -is- how they look. I mean, look at Yoko from TTGL - no one ever calls her a loli, but she's 13 at the start of the series...
Heh... what interested me the most about that discussion was that apparently the Japanese don't agree on whether Azusa and similar characters are lolis or not either.
Favourite parts for me would be Kana (who I'm a big fan of) getting confused about the English word "sister" and the bit where they talk about Yuuki Aoi being as loli as you can get, because, well... she kind of is: http://hashihime.blogspot.com/2011/0...ami-saori.html
The height difference is pretty substantial despite her wearing heels.