When is the best age to tell your child the truth about Santa?

I've seen 12 year old children being teased from their classmates cause they still believed in Santa...In younger ages I find it quite sweet and magical but a 12 year old is old enough to know how things are...When do you think is the proper age to say?When did you tell the truth to your children?
Update: Thanks everybody for your answers
(except mysticalsunshine06---->what is wrong with you???).Since it's hard to find the best answer for this question, I'll let the people decide for me.Thank you all again and Merry Christmas by the way!
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