I don't even remember when I first found out they weren't real. For us, Santa was fun, but he wasn't Christmas.
That's because my parents limited Santa. Santa filled our stockings (with fruit, a little candy, and a few small, inexpensive gifts). And we each got one present from Santa. And it was never our "big" present - usually a game we had really wanted, or something like that.
The kid-friendly explanation - Santa had to provide gifts for all the kids across the whole world, so he couldn't afford to bring everything we wanted. So he just gave us one present, and left it up to mom and dad to give us anything else.
The explanation from mom years later: Dad worked way too hard for the money to buy those presents to let some fat man get all the credit.
And I don't think any of us kids ever REALLY believed in the Easter bunny. A man who could travel to every kid's house everywhere in the world in a single day and leave presents for all of them was stretching it, but at least somewhat believable.
A giant rabbit that went around filling up baskets with goodies for kids was just too much to believe - even when I was 4! I always kind of knew that was mom.
I was especially suspicious given the fact that the Easter Bunny always left three Fanny May Easter eggs in each of our baskets; two were the flavors we kids liked the best, but the third was always a fruit & nut egg. None of us kids liked the fruit & nut eggs. Mom did, though.
Mom always explained that the Easter bunny must have gotten mixed up again about the flavors we liked - and not to worry, she would take care of the fruit & nut eggs for us. Yeah, nothing suspicious about that!