Damian first introduced at 10 and at Rebirth he's 13.
Batman and Robin usually declared to take place for a year, so by the time he became Bruce's Robin he should be 11.
A year passed from the beginning of New 52 until Death of The Family as mentioned in that story so he's 12 then.
After that, there's no significant year passing that I remember being mentioned so I'll go straight to Rebirth.
That's my timeline unless someone wants to say more than a year passed between Death of The Family and Rebirth, like for example, the time Batman Eternal took place.
In which case I'm gonna count Damian's death that took place a week after Death of The Family, as mentioned in New 52 Nightwing, until his revival after Batman Eternal, as an uncounted year because he died and therefore doesn't age, so he'll still reach 13 by Rebirth.
Although Eternal shouldn't take a year because Superwoman was already pregnant before that and gave birth quite a while after Eternal.