Therapy dogs also usually require insurance (or its highly recommended). Also, their training must be outside the realm of what a normal dog would do. For example, an emotional support animal (ESA) is not considered a service animal because what they're "trained" to do is basically be a good pet - which any dog will do (if treated well etc.), love their handler, etc..
But, there is also psychiatric (sp?) support animals which help with severe mental issues like post traumatic disorder and are considered service dogs (these are also the *only* ones that require documentation - usually a doctor's note / prescription).
Therapy dogs are not service dogs because they mainly just need to be friendly but they do require a PAT (Public Access Test) certification (in a nutshell an advanced CGC). As Yuki mentioned, they can go where they are invited (hospitals, schools, etc.)
Yes, 10 week old puppies cannot be service dogs but they can be in training (IMO extremely important to start as soon as you can - Luna's "training" started as soon as we got her but early on, until they can physically do the work, it is exposure, socialization, obedience, exposure, socialization, obedience, exposure, socialization, and obedience.
When she is older (and after her elbows and hips are checked) she'll start with the actual support and mobility assistance training.
Luna is doing extremely well with her training. She's a little stand-offish still but far better than we expected. She's getting the obedience down and her stamina! WOW! The only other dog that out-worked her was Kiri (but she's 4 now and has a well insulated coat). It was a hard week for all 3 of them and they all did well. (Yes, Zorro did well, too, but he's getting up there in years, he's 8 now so we probably have at least another 3-5 years of him now, most of that should be full time (he is the primary now).
We started to do this with Pollux, his physical build would be awesome for it (he's big and strong) but he washed out because he was way to skittish (we may give his training another go in a few months).