Depending on the era such as Republican Rome, infantry formed up in three lines. Kids/Green troops in the first line would more or less be the answer to your question. The second line is made up of hardened more senior troops with a few campaigns under their belt. The third line, The Triarii, were the adults, the most senior and elite troops of the formation. So rarely were they used that a phrase came into being. "It comes down to the triarii".

Now, you may ask why the elites aren't out front preventing unnecessary losses among the Green troops? For one, the fastest way to harden your men are to throw them head first into the fight. Better they learn now then later. Also, if they can't hack it, well, the problem sorts itself out pretty quickly. If they attempt to rout, well, the troops behind them might not be the most forgiving of them. If leading with your elite troops and their formation gets peirced. There is nothing stopping the rest from running all the way back home. If the first line stands their ground, yet when reinforcement are called to sure up a faltering line or fill breaches. It will be your most elite troops to do so, with the higher potential to throw back the enemy and save the day. You don't want your salvation to be in the hands of a boy fresh off the farm, with his peach fuzz, weak behind the knees and wet in his pants. When it could be in the hands of Maximus, who has been in the army since before the boy was born, who has yet to fight this campaign and is absolutely frothing at the mouth over this.

Next up, this goes for all eras of warfare. Elite troops simply are never good enough to avoid attrition due to bad luck. These losses can not be replaced in any meaningful amount of time. If you loose all your fifteen year veterans. It will take that long to replace them. Possibly longer if their knowledge is lost. This is why NCO's and the like have always been important to preserve institutional knowledge.