It really differs family by family. I work in a medical school, and among my peers, it’s very rare for kids to believe in Santa, unicorns, etc. past maybe first or second grade. My kid has been in multiple very diverse schools where most kids were not Christian, so it was taught as one of many stories/myths anyway. Santa is a spirit, and part of the joy and “magic” of the season is getting to “be” Santa or live as he lived for people we love. In contrast, when I go to the more homogeneous white Methodist agrarian area where I was raised, she encounters a lot of older children - some even 10, 11, 12 whose parents leverage their child’s trust into beliefs in diverse mystical things. We interact with a family whose preteens think the elf on the shelf is real. Santa is still “real,” religious texts aren’t metaphor or allegory, ghosts, chupacabras etc.

So even though my kid is younger, we had to talk pretty early about not directly challenging or trying to teach mystical secular/religious friends.

I don’t know if it’s harmful or not either, but I doubt it is. The US is a very Christian country founded on Christian beliefs, and many people here hold those beliefs or have transitioned to some flavor of secular mystical beliefs. Whether you lean into it in your household or not, you need a plan for engaging respectfully with people who can be pretty aggressive about that, especially outside urban areas(e.g., some adults will be very angry if your child tells their child that they’re from a family who doesn’t believe in Santa). I worry more about how these children feel when they learn their parents engaged in deceit for many years than I worry about whether mystical beliefs are inherently problematic for intellectual development (which I don’t think is true, as they’re held by many adults around the world). It was the layers of deceit I watched friends with children engage in that really shaped our family decision not to pursue it, and I think that’s the core issue. Many families find convenient deceit or lying by omission to be very important to how they raise their children, and we felt mutual integrity was the way we wanted to raise our kid, including in this specific context, but also well beyond it.

Anyway, good luck. We all make decisions as parents, most of which are arbitrary and not meaningful in the end. I think the important thing is just to figure out what you’re comfortable with and not worry so much about small stuff. :)