I've come to that age before in single player but haven't messed around too much,saving the mystery for another time,possibly to play with a friend together.That's why I don't really know what's to be expected,really ^^;
I've also observed these differences from my first single-play though:
Single Player:
-The current was there when I first got to the age.I had to jump from very far to make it to the "lighthouse" (or watch tower).In DI,there was no current at first and I could easily get to the lighthouse.
-I also remember there was no door for the lighthouse in SP,but there was a pattern similiar to the one in the Watcher's Sanctuary.On DI,I can enter the lighthouse but couldn't find the pattern.
-Lastly,when I linked back to the age it put me to the backside of the "castles".I hadn't found this place before in SP so I cannot compare.
Shock: Could it be that I needed to reset the age!? O_O