If it's your own choice, I'd say between the ages of 26-40. If it's a forced draft, I'd say it depends on your country and their government.
If you choose to serve your country, then knowing there is a chance you will come out of it scarred for life physically or mentally, IF you come out of it alive, I think that 18-25 is just too young for most people. At that age, death seems more an impossibility, and they are just starting out on their journey as adults. To be able to say that anyone who is 18+ can make a rational decision that will affect the rest of their life is wrong, and most don't realize it. If the terms were shorter, like 6 months - 1 year at a time, I could understand it. that gives them a chance to see if the service really IS for them. Unfortunately, Uncle Sam wants people to make commitments without being fully committed to their soldiers in return!
By age 25-26, adulthood has been reached, and the likelihood of marriage, family life, children, and career or job has been considered by most people. From that point to about 40, people are generally as fit as they are going to be, and JUST STARTING to decline. You no longer feel immortal, but that death is a real possibility, and you can approach the decision rationally. You have had a chance to see the shape of your country and the way the government operates, and can make a reasonable decision on whther or not it serves the country's interest or even your own, in serving it!
As for a draft....or forcing people to fight for things they don't believe in, I think it's wrong. I refuse to condemn soldiers, though...it isn't their decision, it's a pencil-pusher in Washington, who has probably made sure his children are safely tucked away in Canada or have flat feet! I think that a person forced to fight or serve is not exactly the best line of defense...unless somebody is shooting at them, they will be reluctant to work as hard as someone who actually CHOOSES to be there. If I truly am protecting my country and my family, I'll do what it takes...if i'm standing around in some giant litter-box staring at terrorists humping camels and being shot at,let alone having rocks thrown at my head for supposedly trying to "help" people why do not WANT my help, watching my friends get killed by some freak with a bomb strapped to his infant kid's back running towards them....all because some idiot behind a big desk listened to some other idiots whispering about non-existent "weapons of mass destruction" and decided to play "cowboys and arabs" with the guy who tried to have his daddy killed (and failed, even!)...then HELL NO! I wouldn't stand for that, and would rather be jailed than forced into service. You'd think we'd have learned something in Viet Nam! Yet, they still think it's smarter to lead our soldiers from the REAR, behind the safety of a desk!
sorry....strong feelings on that one,lol! I appologize.
What time is a good time to think about starting your own business rather than working for others?