My best advice, put just a small pinch of kosher salt in the carafe before brewing. It will help tamp down some of the bitterness, if you don't like that
I am at the point where I no longer answer my phone if it isn't one of my contacts. If it is important, they will leave a message. I just press the hang up button on my watch immediately, if it shows an unrecognized number, local or not.
I think any age you are comfortable with. My son's school just started using Khan Academy as a supplement. He is in second grade, but third grade math. He really enjoys it, as do I.