The etymology of Adult means "fully grown." I believe the gp of this comment is using the word to mean fully grown in emotional and behavioral maturity in addition to physical maturity. So, perhaps what the gp is saying that physically mature individuals who do these things are not emotionally or behaviorally mature, and so cannot be considered "fully grown." Their arrested development means they are functionally 'children.'
Extended (perpetual?) adolescence is an interesting (recent?) social phenomena.
Extended (perpetual?) adolescence is an interesting (recent?) social phenomena.