The correct answer is: Only argument I is strong.
Argument I is strong because it is based on evidence. It is well-known that the human brain continues to develop until the age of 25, and that people under the age of 18 are more likely to make impulsive decisions and take risks. This makes them less suited for the high-pressure and dangerous environment of the armed forces.
Argument II is weak because it is based on speculation. There is no evidence to suggest that allowing people under the age of 18 to join the armed forces would help the country develop its armed forces. In fact, it is more likely that doing so would lead to more casualties and a decrease in the quality of the armed forces.
In conclusion, only argument I is strong.