Newt's the pink Mew with yellow eyes and orange scarf. He, albeit, a thousand years old, he doesn't sound that way and is more so like a young lad. 

According to Zander, Newt has a somewhat medium voice maybe a little high pitched, but it's playful. cocky like voice. He sounds like an adult, but acts like a teenager a bit unless he's being serious. In other words, tone changes depending on mood, from energetic to stern. 

He is somewhat in the middle, somewhere between mature and immature. He can be serious, but he's also playful. Pretty much not much is known yet about it, so it may be interpreted in any way.