why is 16 called sweet sixteen?

It came from the song "Happy Birthday Sweet Sixteen" by Neil Sedaka. I'm not sure if that's the earliest use of the term, but I think that song popularized it.

Sixteen is the minimum age for being allowed an official driver's license in the U.S.

Thats a milestone and means your on a path to independence.

I guess it is called sweet 16 because the word "sweet" is generally used in a good manner and because it sounds good.

Try this site

Sweet sixteen (birthday) -

A sweet sixteen is a traditional birthday party for girls to have when they turn sixteen years old. In parts of the United States and Canada, sixteen ...

en.wikipedia.org/wiki/Sweet_sixteen_(birthday)

because americains use the word sweet for a good thing also you get to drive in america so thats y it sweet

It relates to leaving an age of innocence, ie the legal age for sex

Youre leaving your innocence behind and becoming a woman, thats all i know.

It's the age of consent for sexual activity or to marry.