guys the description given for austin and kayla in toa is this: 

The cabin door opened. Two more demigods stepped inside. One was a tall boy of about thirteen, his skin burnished bronze and his cornrows woven like DNA helixes. In his black wool peacoat and black jeans, he looked as if he’d stepped from the deck of an eighteenth-century whaling vessel. The
other newcomer was a younger girl in olive camouflage.

so not only is austin 13, but kayla is even younger than that and is probably a greasy 12 y/ old walking around with a bow and arrows