6 months. Generally you can try to start at 6 months of age.
Sippy cups keep the. Cheeks & tongue in a primitive suckling position. Muscles used to drink are also used to talk. Speech/Language Therapists say not to use them for preemies & babies with pre-language delay. If you hold a double-handled open-mouth cup for him at 10 mos., he'll hold it & drink it all while seated when weaned at 12 mos., when maximum recommended intake of milk & milk products is 16-21 oz./day.
6-8 months. Anytime is really ok but most often 6-8 months is a good time to introduce the sippy cup. And water is the best thing to use in it.