As others on the board have said...it depends.
My older daughter was certified when she was 11 and she did wonderfully. in fact, she got compliments from several of her instructors (she did pool work at one location, OW at another, so had a couple of different instructors). That being said, she was very mature for her age, was able to listen well in class, have a long enough attention span to not get distracted during the teaching portion, etc. There were a couple of teens in her class that were getting their OW for a vacation and the instructor couldn't keep their attention for 5 minutes. They almost didn't pass because they continually showed up late for class, didn't pay attention, had a hard time with the skills, etc.
My other daughter turns 11 next week. She is asking to get certified...and we are still talking about it. At 10 she was not mature enough...now...maybe. She did discover scuba a couple of weeks ago and loved it.
Both my children are very comfortable in water...having been on swim team for many years and in the pool every day it is warm enough. I've no doubt about their swimming ability. My concern with my younger daughter is her ability to understand what she is being taught in class and applying it to her diving (i.e. buoyancy control, understanding tables, timing her dives, etc.).
I guess the long and short of it is, only you can know if your child is ready.
Good luck.
John