* He's a teenager: Sonic's personality and adventures are often portrayed as those of a young adult. He's described as being 15 years old in the Archie Comics, but this isn't officially recognized as canon.
* He's been around for a while: Sonic has been around for over 30 years in the real world, meaning he's been fighting Dr. Eggman for a long time in the game's universe.
* Time is different in Mobius: The world Sonic lives in, Mobius, doesn't seem to run on the same timeline as our world.
Therefore, it's safe to assume that Sonic is a teenager in the games, but his exact age is a mystery.
It's important to note that different media might portray Sonic's age differently. For example, the Archie Comics portrayed him as 15, while the Sonic Boom series makes him seem younger.