When, if ever, would you tell your adopted child that they're adopted?

If you were to adopt a child, would you ever tell them they’re adopted. If so at what age?

NB I don’t know what the law is on whether it is imperative that you tell the child or any other related issue so I’m assuming for the sake of this question that there is no legal compulsion to tell the child.

I’d tell them once I thought they were mature enough to understand that them being adopted hasn’t meant that they were any less of family and that biology doesn’t determine relationship.