My oldest is HG, but math is her weakest personal area. She was younger for grade with an early fall/late summer bd and she has skipped one grade. She is about 1 grade up for math achievement right now (performs in the top 10% of her current grade and is in accelerated math), so I'd say, overall, she's about 2 yrs ahead of her chronological age in math. I'd expect that most HG kids for whom math is a strength, not a weakness, would perform higher than her in that area, though.
I also expect that there are plenty of bright, but not gifted kids who are performing 1-2 grades above grade level in math. Dd10 scores slightly above the average 8th grader in math on the EXPLORE (8th grade test) at age 9 (4th grade). She doesn't consistently perform superbly in math (she is subject accelerated one year currently), but she grasps abstract math concepts quite well. I'd say that she does better with much higher level math and not as well with grade level or one grade level accelerated math due to the more concrete nature of 5th-6th grade math. She's maybe MG, maybe HG+ and I do think that math is somewhat of a strength of hers.
