Our son started using HTS when he was 19 months old (he's now 22 months old). We used 7% in the hospital, which irritated his throat at first, but then he got used to it. We use 3% at home because our insurance won't cover the 7%. The 3% takes longer...we get through the 4 ml in about 20 minutes. He is doing great with it, no irritation at all. I really haven't noticed any difference in lung function/breathing...probably because he hasn't had any problems with his lungs yet.
The worst part about using HTS is the added time it takes to do it. We actually added time to his vest so we could get through all the nebs. So now he is doing 30 minutes of vest twice a day. But, in the long run, these changes will only help his health.