That's all I could find too, that Anna is 3 years younger then Elsa.
And yes, coming of age is traditionally at 21.
At least, in our time it is.
In the past, it was much younger, around 15 or 16.
But they generally don't tend to acknowledge that fact in most movies, probably because people wouldn't understand why a girl is getting married at 15.
There was even a historical novel I read once (based on real events) where a grandmother considered it a mark of shame that her eldest granddaughter was 18 years old and wasn't married!
And that was relatively recent, at the end of the 19th century.
But for sake of argument, if Elsa is 18 years old, wouldn't that make Anna 15?