, it's definitely an 15+ age rating, it features cartoon violence, horror and heavy cartoon gore. The game itself is based upon a Creepypasta which makes it an horror story - I won't spoil it but you should certainly find and read the story before allowing your child to play the game.
Sonic.exe is somewhat similar to the Super Sonic of Fleetway's Sonic the Comic series, being a more powerful but evil, possibly demonic version of the character, though that Super Sonic was still Sonic (making him all that much scarier), whereas Sonic.exe is an entity taking Sonic's likeness.