Perhaps he shouldn't have been so STUPID as to go out there and confront the poor drunken kid with a loaded weapon.
Someone goes to investigate an unknown situation on their property. (Hey I'm all for caution and such but this is the real world and you can't just "bunker up" for every noise and event and call the cops every time, it just doesn't work that way).
Someone (reasonably I think) takes along their firearm because you never know.
Someone is confronted by a person who then moves towards them (an aggressive action) despite repeated orders to halt.
We now move into the realm of "MMO".
Means - Healthy young male (capable of doing grave harm)
Motive - Reasonable suspicion of a criminal motive due to time of night, trespass, and non-compliant behavior (remember, nobody knows about the alcohol at this point).
Opportunity - suspect was present and advancing on the defendant.
Seems pretty clear cut to me. Short of being psychic there was no way to know in advance about the drunkenness and so forth and age is irrelevant.
A tragedy for certain but let's keep "problem ownership" where it belongs, with a person who initiated the entire sequence of events because he didn't control his alcohol intake and subsequent behavior.