I never really believed in Santa. At the beginning my mum didn't say much about whether or not Santa was real. She didn't try much to facilitate a belief in Santa, nor did she say that he didn't exist. At some point I talked to her about it and she ended up confirming that he wasn't real, at about 7 I think. Since I've been an adult, she's told me that finding out that santa, the tooth fairy, etc weren't real had quite an effect on her, and I guess she felt betrayed by being lied to all those years by her parents and didn't want to do the same thing to me or my brother, and I appreciate her approach, because I don't think it should be normalised for parents to lie to their children that way. If I have children, I'll likely take a simolar approach.
Probably wouldn't have mattered if she'd told me or not in terms of whether I'd figure it out, because when I was 9 it occurred to me that there were differences in the way santa was depicted on Christmas cards and on tv, etc, and that if Santa was a real person, there'd be no reason for those subtle differences in depictions to exist. In some depictions he had glasses and others he didn't, in a couple he didn't even have a moustache, etc.