I bit the bullet and told my DD who is about to enter yr 8 (yes, YEAR 8, so nearly 13) a few months ago. She's a bright girl, top of the class, no special needs, had obviously heard playground rumours, but still believed, because she wanted to still believe. Honestly, it's not that unusual for children to start secondary believing.
After a couple of years of being deliberately slightly careless, of dropping subtle hints, I finally decided I really did just need to tell her. She was slightly disappointed at first, but very reassured once I pointed out that I needed to know that she knew, but we won't be mentioning it again, and everything will carry on as usual!
Next year I will tell her younger brother who has been chomping at the bit to get me to confirm he's not real for a few years!
Different children are different, but there does come a point, I think, where you have to let them know.