Personally, if you’re shorter than 5′ 8″, leg length less than 30″, or thereabouts you starting to have to make some interesting design compromises to get a 29″ MTB frame to fit properly, so I’d say those numbers are a reasonable lower limit for practical purposes.
With very rare exceptions, 29er frames designed for persons shorter than that also seem to introduce some, er, rather interesting design features and, to my mind, look f’ing awful as a result of a few compromises too far. It’s not that 29er frames cannot be designed for shorter people, rather that they never look as nice as a 26″ wheeled bike designed for the same rider. YMMV.