@hypo
Huh... didn't even think about that. But for what it's worth, I'm fairly certain the Japanese both says 10yo and 12 years late. The '12-years late' thing is the chapter title, so there's not extra context to use, but if I had to guess, it's referring to a slightly different thing. In this world it would seem that you have to start training your magic young (presumably what the starting at 10 refers to), because you'll hit your growth limit at the age of 20 (pg. 30). He's in his 30s. So basically his 'start' is 12 years after his 'growth limit'.
*edit* Looking at it more, perhaps it's more accurate to say, not necessarily at 10, but in the 10s (i.e. 10-19)
*edit* I changed the lines from 10 -> teen/before 20. I think that's more accurate. Thanks for getting me to relook at the raws