My thought is that most 18 year old’s are immature this is a mature decision. I want the person to take the time to think about what they are doing. I would say you are right about them not being able to vote. until they declare there should be no participations in elections, serving in the military or sitting on juries. I would say there rights with in the law should be the same as a foreign visitor. they just have the ability to fill out the paper work during the that time for full citizenship and renouncing the foreign citizenship.
Japan recognizes dual for kids until maturity. In Japan it’s 20, but the maturity age is different for a lot of things there. 18 for some, 20/21 for others.
My daughters are right at that age now, but they’ve already decided to keep their US.
Only reason my wife hasn’t gotten her citizenship is she’s afraid of what she would do if I passed away and wanted to be able to go home if she needed. Now that we’re getting older she’s planning to wait till 55 when the requirements to apply are less.