If you think people can be too young to think clearly because their brain is far enough from a healthy adult human’s as to represent one unsuitable for decision making you necessarily must draw the same conclusions around aged brains.

What I am saying is that all of us, no matter our age, are affected by multiple, complex factors and that we shouldn't be too quick to generalise. Our body does wear down but this doesn't have to affect decision-making. Some go to their deaths in their 90s with a very lucid mind, some start getting dementia symptoms in their 40s and some a lot earlier, including childhood. We are all affected by a society which tends to promote health-reducing habits and environments for reasons of profit and then there are genetic factors in the mix as well. There's quite a bit of research at present on healthy old age.

From the moment we are born, we are one day closer to our death but that doesn't mean we must accept how we fare according to general trends. Our intelligence changes, not only declines, and we can play a conscious role in this.