It can mean an aged enemy, but more commonly it's used for someone who's been an enemy for a long time. It does not mean 'former enemy', which is used for someone who used to be an enemy but is not one anymore. 'Former enemy' translates to 'Były wróg', so 'He is my former enemy' would be 'On jest moim byłym wrogiem'.
I guess this could depend on the context, but I would say it means that he has been my enemy "for a long time" (and still is my enemy). I think this would sound more natural in the sentence "On jest moim starym przyjacielem" ("przyjaciel" = "friend", depending on the context this could mean "fast friend"/"friend from school (with whom I had no contact for a long time, but still have warm feelings for him)"/etc). With "enemy" I would rather use the more epic and sinister "On jest moim odwiecznym wrogiem" (meaning "He is my enemy and has forever been my enemy", "odwieczny" = "eternal").
"foe" is more literary and is used as a polite way to say "enemy" in places where the latter could be considered offensive (like in the news). I'd say "foe" fits in this sentence just fine, but to me it would imply a political enemy, whereas "enemy" could be any kind. "wrogiem" apparently can be either.