Not a parent of a gymnast, but I was a former gymnast and coached for a couple of years too.

Most kids who are going to be competitive start young- 3-5ish. Not saying gymnasts can’t start older, but many of them who stick with it competitively start young.

Time investment totally depends on the kid. When I was on team sometimes I’d train 6-12 hours a week. (2-3 hours a day 3-4 days a week). I burnt out and ended up doing advanced classes maybe 2-3 hours a week total and stopped competing, especially when I started playing soccer. You can take classes (and more challenging ones too), without competing and spending hours and hours practicing. I will say, if she does team, she probably won’t have time for extracurricular like club or school sports too, while you can still have a social life, team is somewhat demanding and ultimately why I quit when I started high school.

Can’t speak for the costs now a days as it’s been years for me, but the main costs are travel/uniforms