Don,
There are a number of good tutorials on Linux and Ubuntu in general available on the web. There are also a number of Ubuntu books available as well, but in my case I would read them for a few days and on the bookshelf they go only to be rarely opened again. Worse, they are out of date when I do need the information, so it is just easier to search the web for the answer.
The Ubuntu community has a wealth of information.
http://ubuntuforums.org/forum.php
I would suggest Ubuntu forum for Ubuntu specific questions.
As for the Beaglebone do expect somewhat of a bumpy road for someone that is new to Linux. It is quite different from Arduino, although there are some really smart folks that is trying to make it somewhat similar and easier to use. Technology changes so fast these days and Linux is no exception. The latest changes to the kernel has upset the applecart so to speak for Beaglebone tinkerers, but it will get better soon. Hang in there.
An now for the "old" part. I am nearing retirement but in my case I have been working with various OS and programming languages for over 30 years. The biggest hurdle for anyone, young and old these days is trying to learn in a ever changing environment. Just I am about to get a grasp on a concept, lo and behold there is something new. Even though I would agree with you that it becomes more difficult to pick up new concepts as we get older, but I'm convinced that it just means being a little more tenacious to forge a head.
Good luck,
Mark