Is preventing an opponent from castling considered an advantage? All else being equal, if you have forced your opponent to move their king, thus preventing castling, would that be considered an advantage?
It'll depend on the rest of the position.

If it's relatively open, chances are you can keep the opponent's king in the center and launch an attack. This might well be a decisive advantage; preventing castling on both sides can be much harder than just one side or none at all. It's worth noting that 'artificial' castling (e.g. Ke8-f7, Rh8-e8 and Kf7-g8) takes three moves instead of one, though the rook might end up on a better file than after a regular kingside castling.

If the center is closed and stable, players often don't even castle at all. A king's move to the second row might be enough to connect the heavy pieces. In such a position, being unable to castle is no disadvantage at all.

As @user1583209 notes, in the endgame, the center is usually the place to be for the king, making castling an unattractive option. In general, the fewer pieces on the board, the less of a disadvantage it is not being able to castle.