No, I don't think any rational person in modern society would consider an 11yo an adult. There are countless other factors to consider
But that's tacit acknowledgement of the fact that hard biological boundaries as a decider of human development are a bad idea
[Hint hint]
/21
Point is, it honestly doesn't matter what issue it's being applied to, the whole 'Those under 25 have underdeveloped brains' argument doesn't hold up any way you slice it.
It's often just an easy way of dismissing the valid perspective of younger people. Which isn't good.
/end