12, then.
And that's part of why they shouldn't stick with him becoming Robin at age 10.
True enough. That said, that's a retcon I could live with. It's not like having Bruce's first encounter with the al Ghuls take place earlier in his career would hurt anything else. Not “before Bruce became Batman”, of course; there needs to be enough time for Batman to establish a reputation.
Let's see: according to Batman Year 3, Tim was around 4 or 5 when Dick first became Robin. In A Lonely Place of Dying, Tim was around, oh, 13 or so. So that's 9 years from Dick becoming Robin to Tim starting his training (which, incidentally, puts the duration of Dick's time as Robin in the 7 to 8 year range, depending on how long Jason was Robin). I don't think that Tim began his Robin career until he was 14 or so; there was about a year of training before Tim actually donned the mantle in his first miniseries. By the time his ongoing series launched, he was 15: old enough to be given a provisional driver's license as an exception to the usual age requirement (so, younger than 16, but not much younger). By the time he became Red Robin, he was… 20, I think? There was enough time in there for Stephanie to get pregnant and carry the baby to term, followed immediately by a one-year No Man's Land (she gave birth between the Cataclysm and the start of No Man's Land) and another “One Year Later” time skip after Infinite Crisis; so Tim's time as Robin needs to be in the 5 year range. If Damian was 12 when he became Robin (which coincided with Tim becoming Red Robin), then he was born when Tim was around 8; that would require his conception to be around Year 5 or 6, when Dick had been Robin for two or three years.
If Dick was between 18 and 21 when he became Nightwing, then he was between 10 and 14 when he became Robin. I think 12 or 13 is likely. That puts him about ten years older than Tim. And if Tim is around 20, that puts Dick around 30. If Dick became Robin in Year 3 and Bruce was 20 in Year 1, then Bruce is 8 to 10 years older than Dick, meaning that he's now pushing 40.
Of course, that's if we had a single, consistent timeline for everything. We don't.