I know it probably differed from village to village, town to town, city to city, but what was the standard bare minimum age for anyone to join the local militia or receive any sort of combat/survival training back in the middle ages? You know, besides something high class like squire training. I'm talking among the commoners.
Only England had a tradition of militias. There was no such thing on the Continent. Service was compulsory for males aged 16-60, although it wasn't enforced much unless there was a looming threat. There were various rules and exceptions at various times for groups such as Catholics (after the CoE split off) and the poor (to keep them from owning weapons that would allow them to hunt on the land reserved for rich people).
In the period you're talking about, I think the main weapon was the longbow, and people would train with the longbow from an early age.