score:13
Basically, the time to put veterans in the front line is when you are in an attacking mode. That is, you put your shock troops in front so that they can actually deliver a shock.
In a defensive mode, you put your less experienced troops in front 1) to give them experience and 2) to have them absorb casualties and spare your veterans. If and when the enemy breaks through, your veterans are available for a counterattack (the shock phase of the battle).
In the "olique" formation, generals would put their best troops at the "cutting edge," and "refuse the flank" with their weaker (backup) troops.
Upvote:2
The Greek phalanx often deployed the most elite soldiers on the right flank of the formation.
Hanson writes as follows in Hoplites: The Classical Greek Battle Experience(emphasis mine)
Each individual hoplite carried his shield on the left arm, protecting not only himself but the soldier to the left. This meant that the men at the extreme right of the phalanx were only half-protected. In battle, opposing phalanxes would exploit this weakness by attempting to overlap the enemy's right flank. It also meant that, in battle, a phalanx would tend to drift to the right (as hoplites sought to remain behind the shield of their neighbour). The most experienced hoplites were often placed on the right side of the phalanx, to avoid these problems.
The most expirienced soldiers would not form the whole front row, but the most important part of it.
Upvote:3
If your veterans are broken, your inexperienced warriors cannot help. Once the inexperienced are broken, the veterans can help. This is simple. If you put veterans in front row, you can rely only on them, and the young warriors are completely useless.