I think that the army takes people as young as 18 because they still don't have their personalities formed so they can be more effectively molded into a good soldiers. However the french foreign legion accepts recruits with ages ranging form 18 to 40 year. The only thing that comes to my mind to explain this is that they would lack candidates in the legion with greater age restriction (because they have a selection process on which they rule out those that aren't either mentally, cognitively and physically capable, based on their criteria). Even taking recruits with more age. in my opinion, the legion is one of the most feared and efficient military forces in the world.
Talking about smoking age, it is a very difficult question...The only thing that I know is that we most of the time intuitively know what is best for us.