You seem like you know US legal practice, so if I may? What would be the legal standing if we attempted to use mutagenics to create mages, with the multiple minors beforehand to minimise the potential for catastrophic failure? I'm guessing it'd be a "fully informed consent" issue, but how does that apply to experimental procedures with potential for life-altering side effects?
Lots of Law and Order, some political savvy, and good Google searches
In this case, I assume you're talking about non Parahumans? Then we'd probably need parental consent to perform any surgeries or invasive procedures (similar to piercing or tattooing). Depending on how the law falls on it drinking a mutagenic may be classed as a medical / cosmetic procedure, something akin to alcohol (delinquency of a minor) or there may be a Parahuman law this falls under which doesn't exist in the real world (akin to someone being Enhanced like the SH9 got enhanced by Bonesaw). If it counts as a medical procedure then legally we could be allowed to use them in case of a life threatening injury that could be healed by it, but otherwise requires consent. Youth Guard can't touch us either way once we prove they're not Parahumans but Magical, but the parents could bring criminal charges (probably Contributing to the Delinquency of a Minor).
For a Parahuman they seem to count as two separate entities at once, one being their civilian ID, which follows non cape laws, and a Cape ID, which
probably follows closer to an incorporated business than an individual. Under the Cape ID minors don't seem to be
legally held as minors, probably due to the risk of outing someone when asking for proof of age. (How old is Rune? Nobody knows, she could be anywhere from 14 to a young looking 24, it makes sense to just legally treat her as an adult whatever her actual age is unless she unmasks to prove it). Presumably this means that legally they could take the mutagenics as a Parahuman no matter their age, once we give them a device that shuts down their parahuman powers the waters become VERY murky.
I can see that going one of a few ways: Easiest is if they've unmasked, then their Cape ID and Civilian ID are effectively one in the same, and they retain the Cape benefits even in civilian ID. Thus when they become a Mage instead of a Parahuman they can retain the Cape ID benefits. This has the downside of requiring them to unmask as well as they need to retain their Cape ID to keep from losing it. Rebranding might be possible if phrased as a renaming instead of disincorporating and reforming their Cape ID. This causes issues with the PRT owning the cape name Vista, so Missy couldn't do this but our villainous friend could theoretically do this.
If they DON'T unmask then Rebranding is easier (just disincorporate and get a new Cape name. In Vista's case the PRT gets to keep Vista and all related properties [IE. merch sales and Image], but Missy can assume a new Cape name.) but they're legally a minor between the time of disincorporating their old Cape ID and forming a new one, so we're back to civilian IDs.
Edit: I'm also not sure how giving a mage a Device would rule, on the one hand it's a thing, probably similar to giving someone an iPod touch with a lot of educational software, on the other it snips Parahuman powers, which could be anywhere from Assault to Practicing Medicine Without a License (yea, a prosecutor could go there). Non parahumans don't have to worry about the later, but Templates could be affected by all sorts of laws.