My girls for example could have probably done work that was +1 to +3 years in elementary, but I'm fairly certain I would have regretted that come high school. My oldest daughter was an early K admit and while K-8th were always fairly easy for her, she's been quite challenged by the top level high school load. I don't regret the early start, but I'm really glad we didn't push for more at some critical junctures.
This seems common in the case studies in the math talent book. Years ahead in elementary, then one or two ahead in high school.
