Pretty simple suggestion: Include a line of text in the "Retiring Your King" collapse, probably under the "Choose Male Heir" dropdown, that indicates a lion under 3 years of age will be aged up to 3 years old.

Reason: It is a pretty rude surprise to someone that has kinged a young lion for the first time to discover that they have lost almost 2 weeks of life from their king, which could have been spent preparing him with patrols.