Please don’t draw parallels between this self-serving nonsense and trans identities.
You haven’t linked the CNN article you mention so it’s difficult to respond to your post, even after mentally stripping it of nonsense.
We tend to end up with defined ages for eg driving, consent, criminal responsibility, etc because it is difficult to use more nuanced criteria. In practice, the law does (or can) take nuance into account (except for things like being able to have a driving licence or legally buy alcohol). Sometimes that is for good, humanitarian reasons (eg an adult with learning difficulties who cannot comprehend the consequences of their actions) or for misguided, vengeful reasons (eg trying a child as an adult because of the severity of their crime), or just plain prejudice (eg treating Black and/or poor children as greater threats than white, middle-class children).
There’s no easy way to draw lines, and no easy way to allow nuance while excluding prejudice. But “whatever the accused decides is convenient for them, personally, right now” is never going to be a criteria, for obvious reasons.