My son started speech therapy for a variety of 'missing' sounds at 4 years old through a private clinic (vs school based).
After about a year, the "s" was the only missing sound. They gave us the option of continuing or just waiting to see if it would come in time. We opted to keep going and the sound was solid within a couple months. We've had to practice here and there over time to keep things in place, but there are ways to make it fun.
He's 8 now and has no issues. He even corrects himself through his missing teeth -- it is just the teeth causing the issue, but he's aware of how he sounds and doesn't like it; I think he remembers being teased and having people say they couldn't understand him.
He has a number of friends -- older and younger -- missing their "s" and "r" who have never had speech therapy. Personally, I think it is best to get it done and over with at an early age.