Should I let my child close their door?

Assuming it's a child without any mental or emotional issues or any history of risky behavior you should give your teen an opportunity to close their doors and have some alone time.

Should you knock on teens door?

Parents should consider giving teens age-appropriate privacy and responsibility. The goal is to create teens that are prepared to be functional adults and you won't be there forever to watch over them.

Should teenagers be able to lock their bedroom door?

There are very few reasons why a teenager would need to lock their doors if their parent practices knocking before entering their bedroom. I don't think they should lock their doors but they should be able to close their doors and have some private alone time.