I think that's just a quirk of HackerOne's username system. The username daniel was previously owned by another account (now known as daniel-hamid) which submitted a bug to Adobe. If you go through @hackermondev's tweets (starting in 2018) they are without question a kid (making games in Roblox and Minecraft) and then started to show an interest in hacking in 2020 (which lines up with when they created their HackerOne account). The claim of being 15 years old is plausible (presumably with parents / guardians who are accomplished in technology).

Thanks for pointing that out, I missed the username change.

He'd be 8 when he made the Twitter account, not when he discovered that exploit. Pretty sure there are tons of 8yos with Twitter accounts.