The voting age being 18 was changed in the 70s, primarily so they could more easily justify sending 18 year olds to fight in Vietnam. Prior to 1971 it was kind of agreed that 18 year olds were still basically children.
Even now, though, in every state in the US, you're not a full adult when you're eighteen. You're not allowed to buy alcohol until you're 21. In a lot of states you're not allowed to buy cigarettes. We've decided that, despite being "grown ups", they're still not grown up enough to be trusted with these things.
To be clear, I'm perfectly fine throwing her in prison for the rest of her life without parole, like she would be had she committed the crime in a state without capital punishment like NY. I'm just particularly turned off by the death penalty. I can sit here and wax philosophical as to why but I'm sure you've heard the arguments and it would be a tangent.