Until I read your words "depression is a child's disease...", I had never heard the phrase.

I think those who say that are seeing depression as something else, because they don't understand it's depth. They think of it as something they can relate to, a pouting child or a day of feeling down/blue that goes away. I think there also might be a hidden wish in that phrase, that they hope your depression gets better for you with maturity/age.

You are right--you count, you matter, your opinions are valuable.