I first attempted to CTB when I was just barely 15. Here I am now, 49, again attempting/considering CTB.
For anyone here above X age to declare that someone below X age does not have the foresight to rationally CTB is the height of "do as I say, not as I do" hypocricy. Are we not, by seeking to do exactly as they would do, proving that their youthful exploration of the decision to CTB is showing perspicacity beyond their age?
Yes, the human brain is in a different state of development below a certain age. Yes, a person gains perspective as one ages, and the younger a person is the fewer options other than CTB they might see. But pain is not age-dependant. Nor is clearheadedness. Nor is rationality. And there's a reason for the truism, "Out of the mouths of babes oft times come gems of truth."
No one here wants to see anyone else die, least of all those who are just starting their lives. But we can't see anyone's situation other than our own, and it doesn't matter anyone's age, the paternalism of solipsistic ignorance is out of place on a forum dedicated to individual freedom of choice.
I would —and have— counsel anyone, regardless of age, to examine all other options before CTB. I think that is part of my responsibility as a compassionate human. But I also must accept the imperative of free choice, and if an individual has made their choice to CTB, compassion also demands I offer what knowledge I have to help ensure their death is as gentle and peaceful as possible.
And compassion should never be age-dependant.