How old should you have to be before you can volunteer for military service?

Apples and oranges. One question asks how old must you be in order to make rational decisions. Another one asks if there is a social norm regarding sex between an 18 year old and someone old enough to be their parent (say, around 36 and probably older).
On the one hand, 18 seems to be the convention for recognizing the development of and capacity for reason. On the other hand, whether or not it is legal, many folks are a little uncomfortable with sleeping arrangements that look sort of like mother/son or father/daughter. It may be the sort of implied incestuousness, or it may be confusion about how one generation could find much in commen with another other than sexual fascination. Sexual obsession is usually not looked upon as a healthy basis for relationships in America since that value diminishes with age and is thus regarded as ephemeral and not a solid foundation (don’t forget that the Puritans established the first English colony in America).
Anyway, I am only the messenger. Don’t ask me to debate it.
As for me, I don’t think you should volunteer for service until you graduate high school. That is usually around 18.
I don’t care if a 36 year old dates an 18 year old, but as I stated in your linked thread, IMHO one would have to have problems with self respect in order to reach that far into the cradle.

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I think applicants to the military are screened for suitability, at least in countries where national service is not compulsory. You can’t compare it with sex. At 16–18 years old people are still ruled by the hormone rush of puberty and that affects rational decision-making more on the topic of sex than on choosing a suitable career.