Right out of my head, I could think of the following skills you need to actively learn to put the command line to use:

- tracking a lot of state in your head as opposed to have all state changes directly reflected on the screen.

- memorizing known commands, flags and microsyntaxes (this is not helped by the essay-like writing style of many manpages and the fact that many commands introduce their own auxiliary concepts that you need to understand before you can use the command)

- finding the appropiate new command for a task you want to do

- memorizing interaction between commands (and the shell) to correctly use pipes, quoting and nested commands (find -exec etc)

- "knowing what you are doing". Many commands are built with the explicit assumption that they are to be used by experts only: If such a command is used wrongly, the consequences can be dire.

A GUI provides significant assistance for all of those tasks and provides opportunities for explorative learning. A command line could do the same but in practice doesn't.

I disagree. My 2-year old niece can barely spell correctly but she is able to select the cartoons she likes on youtube using an iPad. How old do you think she'd be until she figures out how to list files in a dir using the cli?

Mind you, no one ever taught her how to use the iPad, she just did.