In my experience, a typical wireless network will not "dumb down" to the maximum speed of the "slowest" wireless client. Instead, the higher bandwidth-capable clients will experience a somewhat reduced overall bandwidth to compensate for those slower devices. However, these effects are usually limited and only take place when the lower bandwidth devices are actively communicating on the shared network.
AirPlay is a protocol. It is not dependent on the Radio Mode and will operate on any Wi-Fi network. Its performance is directly related to the maximum bandwidth capability between the iTunes host and the AirPlay "speaker."
each device always operates at it's maximum speed.
Well, it tries to.
If I am using my new MBAir, I'm hoping it works wirelessly on 802.11ac.
It will, if it is in close proximity to the AirPort Extreme and there are not too many obstructions in the signal path.
I just was wondering if my wireless network get "dumbed down" to 802.11n since that is the max speed of the slowest device on the network?
like does AirPlay operate on a certain 802.11 network?
It depends on the capability of the AirPort Express that you have, and the distance that the Express is located from the AirPort Extreme. If the Express is simultaneous dual band it might connect at 802.11n (5 GHz)....or....802.11n.
Every wireless device that you add to the network will tend to slow things just a bit. The more devices that you have operating, the more the bandwidth will be affected.
So if I have an all 802.11ac network in the future, but have an old AirPlay device, does the wireless network maximum speed get brought down?
See answer above about the network being "dumbed down".