Shauna seems a little younger than Serena and Calem. I figured she was maybe 13 or 14.

Holy shit somebody else on this site that has that same headcanon.

But yeah, Shauna is shorter (as you can see in this image) and less physically developed. She also kind of acts younger. Doesn't always correlate with age, but it can be a good indicator.

Serena and Calem seem to be about 16. I mostly say that because 16 is legal in over half the U.S. and it makes the porn less awkward.