In the Metal Gear Rising Revengeance flyer that was being distributed at TGS, are a few characters listed. One of them is still a silhouette, but the name below it reads ‘Sunny’. This seems to confirm the appearance of the character in the game.
The pamphlet also confirms the year the game takes place in, which is 2018, four years after Metal Gear Solid 4.
That would make Sunny 10 or 11 years old in Rising. Solid Snake will already have passed away by this time, and it was confirmed earlier he won’t make an appearance in the game.