Thank you, mia.

The quote ("increase age of your oldest accounts by keeping them open and active") which I put in my post was actually taken from the webpage of the second link you gave.

It seems, then, that it is generally not even necessary to keep a given account "open and active" since whether you close the account or keep it open, it will continue to age for up to ten years in your opened-cards credit history.
Am I understanding this correctly?