My son, 17 yo, can get to 35 without becoming symptomatic. It's crazy. We haven't found the point where he becomes symptomatic - and I would prefer not to - when he runs outside his target zone it is high 99% of the time.
When I was 9 I went to camp. The day I came home I was abnormally tired and was laying on the couch. My mom tested me and I was like 15... scared the crap out of me!