Double down if the dealer has a 4, 5 or 6 for an upcard.
Otherwise take a hit.
This hand is played similarly to a soft 13 or 14, and for similar reasons. You'll never stand on this total, since you cannot bust by taking a hit, and the chances of getting a good card are relatively good.
Doubling down gets more money in action, which is good to do in this situation, since the combination of a potential dealer bust combined with your potential of improving your hand make this such a good hand to play.