Save volunteering - how would a man end up being in the front rank of a column or a line during a battle? Who would be in the rear line?
In Roman tactics; Hastati or the youngest were the front line infantry and Triarii or the oldest men were the rear line. (I'm logically excluding the skirmishing troops) and they took turns being at the very front, they would keep swapping troops in the very front so they always had a fresh fighter.
However, in Napoleonic times - was this also the case? Were the youngest placed in the front lines (excluding skirmishers and forlorn hopes).
Would the front line be considered a forlorn hope? Would it only be volunteers or the soldiers took turns being rotated? And then would they have a similar system of having soldiers at the front being rotated with fresh troops in the rear (as how could you stay alive for long a the front)?
And what system did the cavalry have?
You might be interested in some of my previous answers on ranks in the Napoleonic Wars. Briefly, only a small part of the army was engaged in the firing line at any one time; the consituent corps and divisions usually fought in two main lines with an advanced line of skirmishers pushing the firefight even farther from the main formations, and armies usually held considerable forces in reserve behind even the second main line.