Originally Posted by MSPeconomist

At best, you can only get into the lounges operated by your own departing airline and its partners or other contract lounge with which the airline has a relationship. At JFK, some terminals are connected airside (i.e., DL's JFK jitney bus between T4 and T2) but AFAIK you can only generally access the terminal from which your flight departs.

Normally it's best to just use the lounge to which your airline sends you. Then if there's a delay or other problem, you're kept up to date on the situation. In some places, the lounge will announce when you should go to the gate to board, but you shouldn't rely on this.