I just want to know, in case there was a war and the General (or the king, or whoever decided this) was selecting peasants to throw them onto the front lines of battle, were there any exceptions they might consider, besides a peasant being too young, too old or too sickly to fight?
I doubt it to be honest but if it's exclusively up to a king, he could make his own exceptions really.
There was no "draft" as such. The king would require each of his lords (this requirement would have been included in the lord's oath of fealty) to provide a certain number of troops, and this "requirement" would be passed on down the "chain of command" to sub-tenants and so to the actual men who would fight. The number of times you read about the required troops falling short of expected... And you only ever see this from the perspective of the lords involved; Duke so-and-so provided 1,000 men fewer than was demanded, so paid £x thousand (presumably so the king could now hire mercenaries)... The actual reasons for the shortfall could have been manifold; the Duke's holdings were insufficient to support that number of men of military age; the Duke's retainers resisted, for whatever reason (during the English Civil War there were movements known as Diggers and Levellers who were substantially early communists); the Duke himself was lukewarm for the King's cause in the current war. It's unlikely that a peasant would be able to afford to bribe the king...