It will take a long time (i guess a about a year for something acceptable) and not even be possible to make it run full hd youtube via the browser. So i wouldn't hold my breath :<|
If you want your grandparents to have a similiar device you can look at the Pipo X7 or Intel Compute stik and the likes. They come preinstalled with Windows 8.1 which you can make look like Windows 7 with Classic Shell easily.
The price is higher but it is really plug and play - i am really impressed with the performance.