Best to start when they first get the device.
Usually, though, I get called in when they've had it for a while, have ended up confused and frustrated, and just haven't got the patience to start with the
naming of parts.
All they want is a step by step list of actions to complete a task. All very well until something unforeseen occurs, and then they're stumped. But still they don't want to know if I try to explain what a browser is, or an email client, or how folder trees work.
I have had some success with old and young - the best 'pupil' was over 90.