How do I remove my birthday from Skype for iPad?
If Skype saved that data to their servers, you would need to contact them or delete that account.
If it only saved to the Skype app, you might be able to delete the app and hope it clears on reinstall.
More specific guidance will be challenging to provide since we don’t know versions of the OS or App or even which country your account is managed. Most big companies handle accounts differently based on legal requirements and whether you disclose you are a minor or adult, etc...