Pencil2D is a very simple animation program. It offers all of the basic features without the bells and whistles of more advanced programs. Features of Pencil2D include 4 layer options (bitmap, vector, sound, and camera), onion skin option for previous and next frames, and all of the basic drawing tools. While the simplicity is beneficial for teaching beginners, Pencil2D lacks two important tools: the automatic tweening tool and the bone tool (for those that don't know: tweening is the process of creating intermediate frames between two keyframes to create the illusion of smooth motion. The bone tool is a recent addition to many animation programs that allows you to couple your characters with a skeleton layer, resulting in life-like human motion.) You can pretty easily tween by hand using the onion skin feature to draw between the lines of two keyframes (easy to follow tutorial here: http://www.youtube.com/watch?v=hxeHIQKLF_o), but it's more time consuming than having the program just do it for you. Synfig and Ktoon are two other open source animation programs that do offer the tweening and bones features, but I personally dislike the Synfig interface and Ktoon doesn't have a mac option.
Pencil2D teaches the basic concepts of animation from a traditional hand drawn, frame by frame stance. Valuable understandings that can be gained from Pencil2D include how frames work, the difference between keyframes and intermediate frames, basic computer-based illustration, using a timeline with regard to animation, and of course story planning and telling.
The pros of Pencil 2D are that it has a very simple and intuitive user-interface, it's extremely easy to install and run, and provides all the basic tools needed to teach animation without the added confusion of more advanced tools.
The drawbacks of Pencil2D include that the program freezes somewhat often, however there is an autosave option in the preferences menu, enabling Pencil to automatically save your work every few minutes. I would highly recommend enabling this feature. Also, the paint-bucket tool is quite finicky when using it on the vector layer and can be extremely annoying.
I would recommend Pencil2D to fifth grade and above. There's no maximum age on a program like this, because anyone who wants to gain a base for understanding animation would benefit from it.
I'm giving a rating of 3 stars as opposed to more because of the annoyance of the program freezing, and also because of the lack of the tweening and bone tools.