@dcjamesii is correct that we ask for birthdate during initial onboarding (on the website) to estimate a basic starting FTP using age/training frequency. There isn’t anywhere to change that after the fact (as Chad said, it’s not in the Profile section) but this is because we don’t store or use that piece of data for anything else moving forward.
Upon initial onboarding, we give you the option of inputting your own (known) FTP or using our estimation tool to give you a ballpark idea of where to start your training and how to set up the workouts so they’re not too hard off the bat. That’s the only reason we ask for a birth date, and once you’ve done any FTP tests, you’ll never need that estimated figure again. I hope this helps clear things up!