I can't help much with how quickly aeration will remove the muck, but I can say that 6" of muck and a 35 year old pond is not much buildup giving the age of the pond and I wouldn't worry about it. Aeration should help minimize the existing muck and reduce future accumulations. It will also benefit the rest of the ecosystem and could be used to help keep a hole in the ice in the winter.
I added aeration to my pond to help keep muck build up to a minimum (among other benefits) because, prior to renovation, the originally 10 foot deep pond was about 50 years old and had silted in to being only 2 foot deep (8 foot of muck). It has too much watershed and plenty of trees dropping leaves in.
If the owner or HOA has the time, money, and dedication...adding an aeration system is a good thing for the ponds, but without the total investment, I'd say the pond is doing fine. A little muck in the bottom is not a big concern.
