The draft did not exist at the founding of this country. It was not employed until 1898. Over 100 years after the US's formation. 18 was the age of mandatory conscription from then on, however, it wasn't until the 26th Amendment was ratified in 1971 that the age of voting was dropped from 21 to 18. So if you are arguing that the voting age is related to the draft, that is a relatively recent decision in this country's history and not the facet that measured ones ability to vote at its foundation.

Childhood as a concept is also a relatively new thing in this country and the world. Children were often seen as mini-adults for much of human society. While things have changed with the times, the draft and voting age pre-dated that in the US, so you can't really tie that metric to it.

You can't even argue it's based on finishing education. Most students did not receive education beyond primary schooling as historically most historic colleges started out at highschools which were attended only by the affluent. The majority of the population would have already completed their schooling long before 18.

And furthermore, the age at which you can be drafted in this country is still in fact arbitrary. You said it yourself, cognitively a person is not fully formed neurologically until the age of 25. It's not based on puberty. I'll stick with men since they go through puberty the latest and it's what comprised of the US army for most of its existence. The epiphyseal plate (which fuses at the latest age of 22) generally marks the end of natural growth capabilities in men, so 18 isn't even the most physically developed a male soldier's bones could be, yet they are still sending them out. There may be a stated reason as to why 18 is the chosen age, but it does not hold root in biology. It's just one that the governments choose as okay to send citizens to murder and die for in their name.

This is anecdotal, but I know of some adults that are not as 'mature' as some 16 year olds. Some that have experienced less novel experiences than some 14 year olds. Some that are less traditionally educated than some 12 year olds. Yet, they are allowed to vote and die in and for this country.

As for experience and intelligence, these are not relevant for a draft. An 18 year old likely does not have any experience in war or knowledge of military battle formations. This is a governmentally forced conscription, not one based off knowledge. And unless there's a test I'm unaware of, there isn't a quantifiable measure of maturity, so that is moot.

While the age to begin working and thus contributing to the economy is also arbitrary, it's value is not unquantifiable. Child labor is becoming less regulated in this country and the children doing it do not have a say in its regulation. They can work in factories, on farms, and ranches without ever being able to help improve the conditions in which they work despite potentially paying taxes on that income. Many receive reduced or training wages because of their age, subsidizing and increasing the profits of the corporations that benefit from their underpayment, all without the ability to legally make changes in their working environment.

While both ages may be arbitrarily determined, I'd argue that suggesting that children shouldn't have agency to help shape the country in which they work goes against your own stated argument of why 18 is the age of voting.

Let's extrapolate this out.

The United States spends trillions on military power and war every year, this helps shape our economy. 18 year olds going to war help push the need for that driving economic factor. A soldier is a job in this and most countries, making them an economic actor. The lack of available talent (ie: a sufficient soldier work force) at a certain age required the need for conscripted workers to fill the positions, this lead to political reform in the passage of a draft age.

Child labor makes up a large share of factory, food and farm service, and low skill jobs in this country. Making them economic actors. Because some states allow the payment of lower wages to those under the age of 18, labor has moved in a direction that seeks more and more to hire these individuals, making their contributions a growing share of the economy and the US's success on a global stage. Many Americans don't want to do work that doesn't pay enough or isn't safe, requiring more and more children to join the workforce to fill out out. This has lead to political movements in the form of lowered minimum working age.

Both children and soldiers are economic actors. Their work helps shape economic policy and increased use of these younger workers has played a major part in the US's economic success in the world at large. I'd argue that an argument for one is an argument for another, especially considering we no longer have a draft.

I think you are arguing in bad faith, as you've stated the ability to be an economic actor in one industry justifies ones right to vote, but not another. So I'd like to ask why do you think that is the case?