Yeah, there’s a lot in your post that I agree with. I know experience is key but that’s something that builds over time (which is why I mentioned that in the poll when I talked about age being relatively). An 18-year-old won’t have as much experience as a 30-year-old and a 30-year-old won’t have as much as someone in their 50s (for better or worse) but to push out the barometer for ‘not being a kid’ to pushing 30 is a bit ridiculous. You’re definitely at least a young adult by the age of 21. By then you’ve had enough experience to be considered not just legal, and assuming you went to uni straight after sixth form/college and just did a standard 3-year course (no foundation/gap/placement years/added on Master’s/whatever), you would’ve graduated. You’re no kid at 21. Maybe not as experienced as you would be at 29 or 35 (again, that’s something that just increases throughout the life span) but what’s more bizarre than what some people at the top of this thread were saying about infantilising ‘teenage adults’ is infantilising people who are actually in their 20s and referring to them as kids for as long as possible. That just doesn’t make sense.

I’d say 19 for Big Brother. 18’s too fresh and naive but by 19 you’ve had at least a year’s experience of being legal and you’re a bit more mature. Plus that was the age a lot of valuable BB housemates back in the day (Vanessa from BB6 for example) were so I think that’s enough. So it’s not just to be 18 but over it, even if only by a year. 19’s still young but it’s reasonable enough and that one year does make all the difference. Many people only resit years at sixth form with hesitance because being surrounded by school-kids and maybe even having to wear the uniform still when you’re almost pushing 20 is just a bit weird, right?
Ditto for a 17-year-old having to redo Year 11 (luckily that’s not something that generally happens outside America). Too much changes between the ages of 16 and 19 to get held back even by a year unless you really need to, and 19’s a mature mature outlier in all that. Definitely still very young but you won’t be the same as you were even 10 months ago at that point.

Like most teenagers, I thought I was terribly well-informed and sophisticated at 18. It's only when you get older and look back that you realise how green you really were.

you treat 18 year olds like children at your peril

Of course they’re not actual children but they’re not full adults either. Legally, yes. Biologically and experientially, not even close. You have 0 real-world experience at 18.