So the answer to your primary question is that there were no widespread rules about minimum age of service. There may well have been local customary rules that were never written down, but we don't have access to them. It's important to remember that medieval Europe was hugely diverse in its customs and very different in many aspects from our modern ways of doing things. Medieval armies were temporary affairs, put together from what fighting men were available. They were not in any sense professional armies, and it's a matter of debate whether we can even talk about soldiers prior to the 15th century.

With that said, there seems to have been a broad understanding that post-pubescent men, not adolescents, were the proper material for war. You can find examples of boys of sixteen or seventeen engaging in battle, but it wasn't terribly common.

To your second question: in almost all cases, medieval lords did not round up the peasants every time a war broke out. I'm not sure where this myth came from, but it's dead wrong. Even in times and places where a general levy was in place (Carolingian France, Anglo-Saxon England), it seems to have only been applicable to the wealthier farmers who could provide their own weapons and equipment. From about 1000 AD, the levy decreased massively in importance, with semi-professionals taking up much of the slack. So for a typical high medieval army, you could expect to find vassals fulfilling military obligations, retinue men following their lords to war, adventurous warriors who attached themselves to some notable for the duration of the campaign, mercenaries fighting for pay, levied men from the towns and shires, but very few unwilling peasant conscripts.