Giants are exciting, great! I think age largely depends on the gym - for example depending on what age they start kids on team as well as how much they prioritizes uptraining at each level. For example, my daughter's Level 3s group was on the older side at 8-11 yrs old, and giants were not happening there. Giants do not appear to be emphasized at her gym until girls are training to move to Level 7.
My daughter's journey with giants was long. From when she started working on them sporadically in straps and with a spot to when she did them on her own for the first time (12 yr old Level 6) was over a year. She had them briefly but lost them before competition season, during which they were barely training (not in her bar routine). It would almost another year before she got them again and could perform them consistently.
Lets see started doing strap bar giants with a spot in Level 4 that was at 8 years old. She did spotted giants on the pit bar at level 6 which would have been about 9 and she did them on her own right before level 7 so that would have been 9/10 years old. I will say mine did them on the strap bar forever and had some trouble there but when she moved to the pit bar she actually got them pretty quick. I think getting them real good on the strap bar helped with a quicker transition to doing them on her own. I think that was one of her favorite skills to finally get its right up there with your first kip people cheer its a big deal.
Got a Facebook reminder just yesterday alerting me to the 7th anniversary of DD getting her giant! She was 13 and about to compete L7. She was among the last in her group to get it and it would be a very dramatic run up to the start of the season in terms of readiness on bars.
DD's first unsupported giant on the pit bar was age 9 (L4) about 6 mths ago. She does them there or on the strap bar a few times a week. She says coach lets her do some on the regular bars but only when he is at that station.
Spotted babygiants on strap bar: starting at age 7-8
Spotted giants on strap bar: starting at age 8-9
Unspotted giants on strap bar: starting at age 8-10
Spotted babygiants on real bar: starting at age 8-9
Spotted giants on real bar: starting at age 9-11
Unspotted giants on real bar: starting at age 9-13