Note: Let's please leave the context of any discussion mostly Christianity free
My two kids, son and daughter both figured it out around 10yo or so, and both had heard it around school and then asked me, so I just didn't lie once they asked me at that age. I had this rule in my head saying by like the 6th grade if they didn't figure it out I was going to tell them just cause in middle school kids can be kinda cruel.
One thing I think makes it harder on kids now is like for my step daughter my wife had used the Santa online, where Santa sends video messages to the kids and is talking to them using their name and you put in stuff that happened during the year and what they want etc so it is wickedly personalized. I think that makes it way harder on kids that have great imaginations since that makes Santa seem so real.
Funny part is my step daughter knew before my daughter, and my step daughter was the one who was a bit bitter and hurt and so we used exactly this to make sure she didn't hurt my daughters feelings by spoiling it. She got into it after a little while and was having a blast helping us do creative things with the elf on the shelf that year before my daughter figured it all out.
All good times, kids are awesome.
I'd be interested to know if there's any age differences across countries too. My kid (in England) knew at age 7 that something was up, and had fully worked it out at 8.
Xmas is a lot more fun now he knows.