We were told 6 mos. Just switch slowing to avoid the runs...I'd start 3/4 old to 1/4 new for 2 days and then 1/2 and 1/2 for two days and then 3/4 new 1/4 old for 2 more and then you should avoid any runs.
Best to wait 12 months at a minimum. Adult food has too much calcium for growing dogs. Feeding adult food to growing dogs increases the incidence of hip-dysplasia. If you want more info on the subject go to www.ilovemypet.com/jackart.html .