A more thorough explanation:

SI SIT LEGITIMAE AETATIS

If he (or she) is of lawful age.

This is a Latin phrase used in historical documents to indicate that a person is old enough to do something legally.

Example 1: The legal drinking age in the United States is 21. If a person is asked for their ID at a bar, they may be asked to prove that they are of "legitimate age" or "SI SIT LEGITIMAE AETATIS."

Example 2: In medieval times, a person had to be of "legitimate age" to inherit property or hold certain positions of power. If a person was not yet of age, they would need a guardian to manage their affairs until they were old enough.

These examples illustrate how the phrase "SI SIT LEGITIMAE AETATIS" was used to determine whether a person was legally allowed to do something. In the first example, a person must be of "legitimate age" to drink alcohol legally. In the second example, a person must be of "legitimate age" to inherit property or hold certain positions of power.