S.H.
Yes, my son sees a Speech Therapist, and this is true for sippy cups versus straw cups. THIS is what our Speech Therapist said as well about the muscle development....BUT SHE SAID IT IS NOT TO BLAME FOR SPEECH PROBLEMS.... instead, straw cups are suggested to those children who ALREADY have pre-existing speech issues, as an "exercise" to strengthen their mouth muscles. But it's not just about mouth-muscle strengthening...it's ALSO about the muscular coordination and motor skills too. Speech is not just about 1 muscle in the mouth, it's a Coordination of MANY different muscles, and the tongue, and the throat, in unison together. My Son's speech therapist suggested a straw-cup for him... BUT my son already uses straw cups ALL THE TIME. So, a straw cup does not "cure" a speech problem either...it is merely used for physical therapy of the muscles.
And as far as speech problems go.... it is not the sippy-cup per say that causes it. A speech problem is very complex, and has to be assessed, and it can be due to many many many different elements, both physical/developmental/physiological/and gender related to name a few.
For my son, my son's speech delay/problem has NOTHING to do with the kind of cup he drinks out of, NOR was it the "cause" of his speech "problems." My son in fact does not have any developmental problems per his assessment...nor any apraxia etc. But he is bi-lingual. In fact, he was assessed at actually being advanced for his age... but he just is a "late" talker. So it has NOTHING to do with the kind of "device" he drank out of.
Now, a straw cup can be used for ANY child, as long as they are able to. And like anything, it takes a baby/child time to fully "master" a task. BUT, I would not call it "training" a baby.... it's just something that occurs developmentally. Like walking. Like sitting up. Like pulling up. Like rolling over, etc.
The thing is... a sippy cup does not "cause" speech problems...and anything introduced to a baby/child has to be developmentally and/or age appropriate. Thus, sippy-cups are a transition object between the breast/bottle and to a regular cup.
There is no one "age" at which you can or cannot use a straw cup... my kids used one from about 9-12 months old. My daughter like it. My son too. They only used a sippy cup for a little while. And a regular cup and a straw cup was used in tandem with each other... versus in a "sequence" one after another. I would just put the "cups" out on a coffee table, both kinds, with water, and then they would just grab and drink at will. I did not "TRAIN" them to use it... it was just a free-flowing natural casual introduction of it, to my kids. No biggie. And then they just get used to it, at THEIR own pace.
All the best,
Susan