I know that if you have a 802.11n network (populated exclusively by 802.11n devices) and then a device joins which only runs at 802.11b or g, it slows the network down to that speed (b or g). Does this also apply to n devices joining an 802.11ac network?
Meaning, if you have a network populated only by 802.11ac devices and you introduce an 802.11n-only device, will it slow THAT network down?
There's some confusion here. Bob Timmons is speaking in a more technical sense, while Babaganoosh is asking for a more real world scenario. To (hopefully) clarify (and I'm going to stay with 11n + 11g, since I'm not yet sure where 11ac improves on things):
On a mixed-mode network (e.g. backwards compatible), devices capable of connecting at 11n will do so, while devices able to connect at 11g will do the same. So long as only one device is communicating with the router, things should be fine. The problem arises with "simultaneous" communication situations. While the details are perhaps overly technical, the bottom line is that a number of network "gymnastics" ensue to ensure that both devices can communicate in a stable manner, without data loss, etc. The real world implications of this can result in as much as 30% drop in speed compared to the theoretical maximum, but that tends to be extreme (e.g. one device is making constant requests for data, such as streaming, etc).
I don't think Bob Timmons is saying an 11n device will experience no slowdown, just that the whole network won't "drop" to 11g (which is a common misconception).
I'm no genius when it comes to 11ac, but my sense is it's no different. The network will have to "adapt" to the presence of an 11n device, thereby resulting in slower throughput than if no such device existed. That said, it will not "turn off" 11ac and run only at 11n. It'll keep 11ac going and just do what it needs to to ensure all works well.