A light HN story... Stamp collecting (or was it the Kool-Aid stand) might've been my first "startup", in early grade school. I casually collected stamps myself, and would get duplicates of interesting-looking ones. So I turned my duplicates into stamp collecting starter kits and supplements, packaged in glassine envelopes with handwritten labels and prices, packed them into my little kid's blue suitcase (traveling salesman's case), and brought them to school, to sell to other kids. I don't recall whether I got any sales.
Another stamps-related early sign of disturbing startup inclinations was when, in early grade school, the class would occasionally be put to work, for a fund-raiser, trimming out stamps from boxes of envelopes. I suppose the trimmed parts were going to a company, to soak off, press, and sell. Well, I don't know how I'd even heard this was a thing, but I also trimmed out the addresses from the envelopes, then presented a pile of them to one of the mothers helping out in class, saying we could use them to assemble and sell a mailing list. I might only recall that because one of the mothers told my mom, probably in a "you'll never guess what your child said this time" way, and I guess it was a mix of funny and embarrassing to my mom.
(After that, there were several non-stamp-related, more-credible childhood "startups", until I lucked into a hardcore software engineering job. They used to say that stamp collecting was a great way to get children started learning history, but it got me started learning business.)