Some people say and consider a 13 year old a tween and they're partially right but in my opinion a tween would be a kid between ages 10 to 12.
When an age has teen in the number when you say it then I would say that thirteen is a teenager instead of a tween.
So between ages 13 to 19 are teenagers even though at 18 you can be considered an adult full adulthood age isn't until 20 and above but that's just my opinion.
I would say that any kid under 12 years old would be considered a kid instead of a tween and then 8 and under would be considered a little kid.