New recruits in the Roman army were called tirones. Their implementation and service could take up to six months before they could be called a real soldier (miles).

Vegetius, a historian from the 4th century CE, described what was guided in choosing the ideal candidate to the army:

Those employed to superintend new levies should be particularly careful in examining the features of their faces, their eyes, and the make of their limbs, to enable them to form a true judgment and choose such as are most likely to prove good soldiers. For experience assures us that there are in men, as well as in horses and dogs, certain signs by which their virtues may be discovered. The young soldier, therefore, ought to have a lively eye, should carry his head erect, his chest should be broad, his shoulders muscular and brawny, his fingers long, his arms strong, his waist small, his shape easy, his legs and feet rather nervous than fleshy. When all these marks are found in a recruit, a little height may be dispensed with, since it is of much more importance that a soldier should be strong than tall. – Vegetius, De re militari

Vegetius also mentioned what the recruiter’s age is considered ideal:

If we follow the ancient practice, the proper time for enlisting youth into the army is at their entrance into the age of puberty. At this time instructions of every kind are more quickly imbibed and more lastingly imprinted on the mind. Besides this, the indispensable military exercises of running and leaping must be acquired before the limbs are too much stiffened by age. For it is activity, improved by continual practice, which forms the useful and good soldier. Formerly, says Sallust, the Roman youth, as soon as they were of an age to carry arms, were trained in the Strictest manner in their camps to all the fatigues and exercises of war. For it is certainly better that a soldier, perfectly disciplined, should, through emulation, repine at his not being yet arrived at a proper age for action, than have the mortification of knowing it is past. A sufficient time is also required for his instruction in the different branches of the service. It is no easy matter to train the horse or foot archer, or to form the legionary soldier to every part of the

drill, to teach him not to quit his post, to keep ranks, to take a proper aim and throw his missile weapons with force, to dig trenches, to plant palisades, how to manage his shield, glance off the blows of the enemy, and how to parry a stroke with dexterity. A soldier, thus perfect in his business, so far from showing any backwardness to engage, will be eager for an opportunity of signaling himself. – Vegetius, De re militari

More about the life of the legionnaire