He built some new ones.
The newest suit we saw in Iron Man 3 is the Mark XLII, but a Hot Toys announcement of a new figurine for Age of Ultron reveals that he's using the Mark XLIII in Age of Ultron. He must be building new suits (along with the Iron Legion, although they seem to be unmanned drones rather than wearable suits), despite trying to start afresh at the end of Iron Man 3.
I don't think we've seen why he's decided to build new suits yet, whether in tie-in comics or anywhere else.
In an interview with Philly.com, Kevin Feige explained that we will see more of this arc:
For example, at the end of Iron Man 3, it looks like Tony Stark has decided to stop being Iron Man, but the beginning of Age of Ultron shows the team together, with Tony back in the suit, with no explanation about what happened in-between.
“There will be storytelling about that [in future films], as a matter of fact,” Feige said. “In terms of Tony's… journey, the fact that he's having a hard time putting down the suit will come into play.”
There are hints that this may be explained in Civil War, but nothing concrete. This may be all the information we have for now.