From as.com:
About your My account and Mileage Plan profile
Setting up a My account profile will give you all of the benefits of our award-winning Mileage Plan™ program, along with many extras that make booking on our site quick and easy.
Children under 13 are not eligible for a My account profile, but may be enrolled in Mileage Plan by calling us at Alaska Airlines 1-800-252-7522 (TTY: Dial 711 for Relay Services) 5:00 a.m. - midnight PT, daily
https://www.alaskaair.com/content/mi...count-overview