I will assume the person you are speaking of is guilty but the death penalty is still wrong because some innocent people will get put to death. There is no getting around this as far as I know. No way to ensure that innocents never get executed. You really can't get around that point.
Accepting this argument though (innocents inevitably will be sentenced) where do we draw the line?
Are we perfectly okay then to take away 10-20 years of someone's life even though we know innocents will be convicted? There will always be mistakes but don't we make determinations like this in all kinds of policy? We allow certain amount of toxicity in water/food/etc., we only go so far in implementing auto safety (don't require helmets for people driving in cars or flame retardant suits) even though we know that this results in a certain amount of death. Now certainly we can't just say, "hey people are always going to die so what's the big deal" but how far do we go with restricting punishment based on the idea that innocents get convicted? For some a 20 year sentence is a death sentence, for those cases do we adjust the sentence downward accordingly?
re: X number of people getting released after being convicted and put on death row, is this really an indictment of the death penalty? The mechanism which got these people their freedom was part of the mechanism of the death penalty process. Also isn't it likely that absent the death penalty many of these people (if not all) would have had life sentences and probably still be rotting in prison in anonymity?
re: the ruling. I disagree with it (for the same reasons I disagreed with the ruling re the retarded) primarily based on the idea that once you convict someone as an adult they should be punished as you would punish an adult. If you decide that they don't have the cognitive ability to understand their actions as an adult would and thus are not deserving the punishment an adut would receive then they shouldn't be tried as adults to begin with.
re: cost v. life imprisonment.
The problem with looking at costs is that the costs are based on the current situation. As noted the appeals alone costs in the millions. But then doesn't that indicate that life sentences aren't appealed? Is that better or worse? If the death penalty were removed would that mean a higher number of life sentences would be appealed thus increasing the cost of life imprisonment?
re: arbitrary age limits
As someone else noted we have to draw limits elsewhere for practical reasons. There are thousands if not more 15 year olds who would be qualified to drive, or 17 years old qualified to vote or 20 year olds qualified to drink but its not a moral barrier which prevents us from doing a case by case analysis but a practical one. This is very distinct from the age categorization that's going on in this case and the argument being made in favor of the ruling is a moral one not an administrative one. I.e. their arguing that executing minors is immoral, not that executing minors is impractical.