At the end of the day, you essentially dealing with two differently sized rectangles. I'm pretty sure most people covered rectangles by the age of 4.
Yes, rectangles, but also logically rearranging several small rectangles within another rectangle in a way that maintains their usefulness. If someone has an entire 1920x1200 screen covered with fences of varying sizes and dimensions, translating that smoothly to a much smaller screen with a different aspect ratio requires some thought, a lot of logistical work, and having to account for a huge variety of permutations. Don't know many 4 year olds with that level of programming knowledge. 
Like I said, dicey, but possible.
