Any freeborn male in relatively good physical and mental health, who had at least reached his 18th birthday was eligible to join the Roman army. The military age was 17 to 46. Therefore, you could join the army at the age of 17.
One must be between ages 17 and 42 to join the United States Army. A parent would have to sign at age seventeen, though. Parental consent is required until age 18. The age limit was decreased in 2005 because more recruits were needed for the war.
When the Romans instituted a professional army of volunteers, any able bodied citizens of sound mind and of military age (17 to 46) could join the army. Freedmen acquired citizenship on being freed. They could join the army, too.
Technically one could join the Roman army at the age of 18. The enlistment ages were 18 to 25. However this was not always adhered to. It was size that mattered as the Romans tried to keep the men at roughly the same height.