DDs are getting on a bit now (DD1 starting senior school!). Although it is obvious they no longer believe in father Christmas, it has never 100% been expressly acknowledged. They kind of like the whole pretence even though they know it is not real - the excitement of the stocking in the morning (from FC - presents from us and others under the tree which are opened after lunch).
We have not directly lied to them about it, at least since they were of an age where we would not expect them still to believe but they have not asked the direct question (although have made some comments in our hearing which mean we know they don't believe but we have not taken the opportunity to expressly confirm this just smiled knowingly etc). we have kind of concluded that because they have not asked directly they want the status quo to continue.
But there must come a time when it is ridiculous still to pretend even though it is a funny kind of pretence where we all know no-one believes it? We would not spend less and would probably still give them a stocking so they have something first thing, but the balance between stocking and other presents would change and we would acknowledge the stocking was from us (to stop him getting credit for my very thoughtful purchases!). It would also mean that I could take a bit less care about the secrecy of buying things and where to hide them and ensuring different wrapping paper is used for the FC presents etc
I cant remember when the pretence was stopped when I was younger, although we were a wide range of ages so it was probably acknowledged between myself and my parents that I no longer believed while as a household the FC thing continued for youngest sister's benefit.
DDs are very close in age and DD2 is more knowing so if anything she probably worked it out first!