Even though that rock has an edge it's unlikely it's sharp enough to be of any real value as a cutting tool. Most stone age tools which were used for cutting were some form of flint which forms a very sharp natural edge when split. That rock doesn't look like flint, so I doubt it was modified as a cutting tool.
That being said, it's not impossible that it's a much more primitive stone tool. The most ancient of stone tools were probably just rocks with coincidentally useful shapes. Rocks which didn't need to be modified at all and were used just as they are. This could have been used as a stone age tool even if it wasn't manufactured as a stone age tool.