What age to tell a child on the spectrum the truth about santa?

i have an 11 year old who has aspergers and a 2 year old who is not on the spectrum. We want to tell my older son the truth but we are afraid he will tell other younger children, and ruin it for them. My 2 year old finding out is not a big deal, as he is to young to remember but other children are. any recommendations?
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