I think I was about 8. My 12-13 year old brother told me I wasn’t getting any presents that year because Santa was dead.
Distraught, I ran to my parents who decided now was the time to break the truth. I don’t remember being bothered - the alternative was clearly much worse.
Aren’t older brothers lovely??
I found out the year I was deemed old enough to stay up for midnight mass on Christmas eve and so witnessed my parents putting the gifts under the tree. Being able to attend midnight mass was kind of a rite of passage, though, so achieving that milestone of maturity offset any disappointment I might have felt.
Since you ask about similar beliefs or customs… I think I was six when I lost my first tooth, and my parents told me about the Tooth Fairy. I mulled over it a bit in my mind, but I just couldn’t bring myself to believe in some magical winged woman.
…So I decide it must be God instead. (Okay, atheists, you can stop laughing now). I grew up in a religious household, so God and angels and all that made perfect sense to me. But anything else supernatural just didn’t fit in my worldview. I believed that God was miraculously replacing my teeth with quarters until I was eight and some kids in camp put forth the distressing theory that it was your parents.
This is basically me. I said “age 8” on the poll, but that’s a very rough approximation. I do remember noticing that “Santa’s” handwriting looked an awful lot like my mother’s. 
I don’t remember being particularly torn up about it, either. I still got presents every year, so there really wasn’t a whole lot to get upset about.
I don’t remember when my brother told me- I must have been pretty tiny, but I do know he said ‘Don’t let mum and dad know you know’…
So I firmly believed until I was about 10 or 11 that if I told my parents I didn’t believe in Santa, they’d just say ‘Oh well, no point in having christmas this year’ and that would be the end of it all.
My dad did tell me when I was 4 that the tooth fairy was actually him in a tutu though…
I don’t remember ever actually believing in Santa Claus. I always figured it was just a story my parents were telling me, for some obscure and incomprehensible grownup reason. I went along with the charade for a while and pretended to believe because it seemed pretty important to my mom.
I don’t remember how old I was but my brother told me Santa wasn’t real and that he was just a spirit.
I pretended to keep believing until I was in the 5th grade because it seemed to be such a big deal to mom to be Santa. Other than that though, I probably knew for sure much earlier than that (say 6 or 7) by finding the room in my Grandparent’s house were the stash was kept.
When I was nine or ten. Don’t remember the exact age, but I voted nine in the poll to limit embarrassment. I was a night owl even then, and was yet to fall asleep when I heard noises down the hall. Naturally, I got up to investigate. I found my mother putting presents in the stockings and promptly burst into tears, wailing “You told me Father Christmas was real!”. She said “He’s real if you want him to be! It’s OK!” That pissed me off even while I was crying because that was not the definition of ‘real’, but hey - she’d been taken off guard in the dead of night at Christmas Eve by a sobbing, accusatory little girl. Comforting bullshit probably seemed like the best option.
I think I was suspicious for a couple of years prior, but I’m pretty sure it was the year I was seven that I recognized my mom’s handwriting on the gifts. I know I wasn’t older, but it might have been when I was six. Either way, not a major trauma.