Depends on the type of fighting being done, especially if there's a confrontation with an enemy filled with younger soldiers. You do lose agility and speed as you age.
Additionally, there's the matter of obedience. If you're being conscripted because you're "disposable" and closer to death anyway, you're more likely to ignore orders you disagree with. Even incarceration carries less of a threat.
On the obedience front I’m inclined to disagree, if you tell a 19yo private to go scale a wall and clear a house you don’t get a lot of questions, if you tell a senior NCO to do that, you get all sorts of questions (most very pertinent) and may likely get told to sod off unless you have good answers!
Additionally, there's the matter of obedience. If you're being conscripted because you're "disposable" and closer to death anyway, you're more likely to ignore orders you disagree with. Even incarceration carries less of a threat.