bright Just seen your Q about PECS, sorry.
DS started using PECS at about 3.6 or so. He dropped it after about 6 or so months when he started using spoken language.
When he started PECS, I took photographs of everything under the sun that was important in his little world: people, buildings, food items, rooms, toys, DVDs etc. He got to Phase 3 within a couple of weeks and sort of stayed there. But he would link photos, so he would give me a photo of a particular DVD plus a photo of his comfort blanket and I knew that meant that he wanted to sit on the sofa with the blanket and watch the DVD.
When his spoken language started to come, he used his PECS less, and by the time his spoken language was easier for everyone to understand (that took a few months), he completely dropped his PECS. So, yes, it was a stepping stone for him. If he had become frustrated with his spoken language, I could understand holding onto PECS for longer, but fortunately, DS wasn't frustrated in this way.
All of the PECS work cam through his ABA consultant. It was never suggested by SALT, despite DS having been under (woefully inadequate) SALT since he was 2.1. We meet with his ABA Consultant fortnightly, so we could be really responsive, and also spent lots of time discussing things, rather than the very limited short slot we would have had with an SALT.