C.A.
My daughter had the same diagnosis by her dentist about 1.5 years ago at age 5. My daughter seemed to speak clearly to me and be understood by others. I didn't realize she had speech issues until I heard her count from 60 through to 79. I could really hear it in those s's. We have been seeing a speech therapist (off & on due to our schedule) and definitely seen progress. The other thing I learned about the tongue thrust is that her tongue rested on the bottom of her mouth and it should be at the top. The first focus of therapy was to strength the tongue muscle and get it to the top. That's not an easy task and it still isn't always at the top.
I would say earlier is better to deal with this issue. Speech habits are easier to break this way.