generally, the right to self defense requires a reasonable belief that there is imminent, severe bodily harm; and even then, the measures you take must be proportionate and reasonable. every state has it’s own nuances, though.
As far as the general laws go… somebody standing on a street corner leering at you? it’s proportionate and reasonable to cross the street. Somebody brandishes a firearm and says they’re going to kill you? it’s reasonable to believe them. (unless you know them, and you know they’re joking. Details. those kind of jokes aren’t really funny though.)
Simple trespass is not itself a threat. The teen was presumably unarmed. At no point was the asshole reasonably in need of self defense.