Final answer:
The exact age at which a soldier can choose their aerobic event is not provided, but it generally depends on military branch regulations rather than age, with more choices typically available to more experienced soldiers.
Step-by-step explanation:
The age at which a soldier can choose what aerobic event they want to take is not explicitly mentioned in the reference information provided. However, drawing a parallel from the historical context of the Spartan military, where a Spartiate man became fully integrated into society at the age of thirty, one might infer that soldiers are usually afforded more autonomy and choice in their roles as they gain seniority and experience. In modern times, fitness requirements and the ability to choose specific events typically depend on the military branch and specific regulations rather than age.