TL;DR: Anyone under 25 should not receive the death penalty. Their brains are not yet developed enough to make intelligent decisions.

In order to be eligible for capital punishment, the person in question should be required to be a minimum of twenty-five years of age. Though legally the age of a so-called ‘adult’ in many countries is set at eighteen, this number has no correlation in estimating the mental development of said person. Those who are of legal age are considered to be at the age of majority but not at the age of maturity. Though their brains are mostly developed by the time they’ve reached age eighteen, they don’t finish developing their prefrontal cortex for another seven years on average. The fact that most eighteen years olds have brains that are ‘mostly developed’ invites the argument that these adolescents are developed enough (and therefore rational enough) to understand the depth of the crimes that they are committing (or at least those heinous enough to warrant the death penalty). This argument is plausible, except for the fact that the primary function of the prefrontal cortex is for controlling rational judgement and regulating impulsivity, the two traits found to be most vital when evaluating criminal activity. Though the brains of 18 years olds are mostly developed, the functions they lack are the most crucial. To claim that developing children should be put to death for situations they may not be able to fully comprehend is an abuse of power on behalf of legislators. Though some crimes are severe enough to warrant longer sentences to ensure justice on behalf of affected parties, it would be unethical to take the life of someone who is still mentally developing and thus unable to make intelligent decisions on their own. Being unable to make their own rational judgements and recognize destructive patterns, these adolescents may be not acting on their own accord but instead imitating the influences around them. Furthermore, children born into less fortunate circumstances may be conditioned to view acts of violence not as criminal but instead as a method of survival. We cannot expect these children to break these habits and unlearn these patterns before the time they are mentally fully developed. It falls upon us as a society to educate our youth, not to take their lives after we failed to provide the resources and understanding needed to guide them in the right direction in the first place.

Many countries, especially those in Scandinavia, completely removed both capital punishment and life sentences, as both qualify as "cruel and unusual punishment", a concept some included in their own constitutions at the inspiration of the American constitution.

As such, everyone who goes to prison WILL come out and be amongst others, so proper and lasting rehabilitation is paramount.

Through proper rehabilitation, they have achieved some of the lowest prison recidivism (repeat offenders) in the world. Complete rehabilitation in less than 20 years. Usually in less than 5.

They have proven that capital punishment and life sentences are not necessary in a properly structured system. Therefore, any life sentence or death sentence in any other country represents only the inadequacy of that country's criminal justice system. As well as lives lost due to such inadequacy.

A heinous crime is a heinous crime regardless of the age of who committed it

I strongly agree on the grounds that I think no one should recieve the death penalty. It is a barbaric and unjust act that prioritises vengeance over justice.
Furthermore, you either have faith that criminals can be rehabilitated into society or you don't. If you don't, then why wouldn't you advocate for all prisoners to be executed and alleviate the strain on the taxpayer? If you do and still chose to execute some criminals then you're chosing to kill someone over rehabilitating them. That's barbaric.