Every character's reactions and actions should be in character and determined by the experiences of that character.
So, the younger versions of the characters are absent the life experiences of the older ones - and that should be reflected in their actions and reactions.
The teen characters should be in and of themselves organic and realistic for their ages, regardless of the other contrasting characters in the book - ie, the character should be written the same way no matter what other characters are there.
So, one approach, if you are having trouble keeping the characters' voices and experiences separate in your head, would be to first write the younger characters' story. And then put it aside and write the older characters' story. And then weave the two together in revisions.
If you used an outline of of signposts of the overall story, you should be able to keep both stories moving forward.
BUT, if your real question is how to make teen characters sound and seem like real, age appropriate teens, then you have research to do - read a whole lot of recent, contemporary, well-reviewed YA novels; go hang out at places teens frequent and observe; etc.
~suki