The US census used 1997 - 2013 as the range. Which gives us an age range of 10-26, making the median age 18 - and I was thinking of 21 as adult, rather than 18 - which is why I said “they’re teenagers or younger” - but yes, you’re right, “only” 40%-ish of gen Z are under 18 :p
Run the same survey again in 11 years and compare 21+ gen Z to 2023’s boomers and I bet the results aren’t even close.