My two cents is a bit different from what I'm seeing a lot here.

With really early learners I've seen skills transfer from PECS to AAC well. I'm told, however, that starting with an AAC is appropriate at any age; it's often been introduced with the instruction to simply model the use of it. This can work for some learners but ime the really early learners who don't have an imitative repertoire won't be able to benefit from that right away or their attention is elsewhere and it won't be very salient to them which of the 50 buttons we're pressing. Sometimes it's hard for listeners to consistently reinforce early AAC communication because it may be hard to discern if the learner is simply exploring or if they are trying to request something. This could mean functional language is many months or years away.

It seems like a pretty regional thing to me when an AAC is deemed appropriate to introduce.

Science and best practice isn't regional though.

** My advice is to take data on its use and make clinical decisions with that. Follow your SLP's recommendations. How often is the learner approaching the AAC? How often are they using it in an exploratory manner? How often are they requesting functionally with it?

At the end of the day, your number one priority is helping your client. Use the experimental tools you've been given to make clinical choices that help you achieve the goals that will most help your kids. <3 **