Sunny27 - I know what you mean the S and the TH get confused all of the time with kids that have articulation issues.
For example the word is scissors will be pronounce as the word thissors because the child will protrude the tongue slightly outward instead of keeping their teeth clench which is the proper form for the S sound.
Most children master this sound by age nine. I know that many young kids go to speech therapy but some really don’t need it.
A 5 year old for example, is really young and will need a bit more time in order to fully pronounce the sounds correctly. This had more to do with their muscles in their mouth developing than anything else.
Now a child that stutters or has trouble even speaking is a different story.