A bit of a background on Shantae, she was created by a feminist and the game is intended to push female empowerment.
On her age, yep, she's 16, because she was born in 2002, although conceived as far back as 1994.
Yeah, some artists draw her older, and other artists draw her younger. In the original game
But to be real, I looked up the person who said she was 16, and it wasn't her creator (Erin Bell). It is unlikely age was a concern of theirs when developing the character. The more recent art of Shantae looks a lot younger than the art from the earlier games, so perhaps her later character is 16, but her earlier art looks ambiguously mid-twenties to mid-thirties.