Just a lurker...in love with Cullen since DA:O.
I was thinking of the various ages of the LI's for a hetero-fem and most have to be near or over 40.
If Cullen was a full templar in DA:O that means he was 18, minimum. However...
Cullen is a Knight-Captain in Act 1 of DA2 which he says (during Keran's quest) knighthood takes 10 years. So in DA2, around 9:31, he has to be 28, minimum.
By the opening of DA:I 12 years have passed since the opening of the 5th Blight in 9:30 as it is now 9:42 correct? So while the math seems to work 18 +12 going by pure time passage since DA:O, it doesn't if he was a "Knight" in DA2.
So by DA:I Cullen is 37ish, maybe even 39ish...unless his title was pure fancy and he wasn't a real "Knight".
*goes back to lurking, though more like devouring every page full of delicious banter, art and comradery*
Yeah, the 30 seems a bit young to me. I still headcanon him as 34-35ish... which I can squint at and call "about 30". 
FWIW, his DAO chr file put him at 24 in 9:30, which I've always found plausible for making him 34-35 in DA:I (which takes place in 9:41). Brianne's new calibration makes him only 19 in DA:O and 20 when he went to Kirkwall and was promoted to Knight-Captain. (and younger than Alistair, BTW. um, yeah.)
As for the 10 years until knighthood, I'm not sure that's quite what Keran meant. He'd been training for five years, and then they told him he'd have another five years during which they'd "watch" him. I always figured that meant they added on to his training period for the "making sure he's not a demon" part. I also wouldn't be surprised if it's a different amount of time for everyone, so long as you've completed all your requirements or whatever.
Finally, one of the Kirkwall templars says he started with Order when he was 15, and we now know Cullen started when he was 13. So, he could have started a bit earlier than normal due to his zeal.