No it isn't. That's precisely why, in fact, a 13 year old making a dumb decision is often given much more leeway or compassion in most people's eyes than a 40 year old making the same dumb decision. I mean, if I screw up some facet of my life in a fairly basic and/or spectacular way, nobody will rush to defend me by saying "hey, give him a break -- he's only thirty!" But (adult) people expect kids to do dumb things, because like I wrote in an earlier post, they were kids themselves at one time. So, in a way, being dumb is a part of growing up, and if not exactly a function of age (since we all know immature whatever-year-olds, I'm assuming), there is at least a pretty strong correlation between getting older and learning things, since you will inevitably gain new experience and with it a new perspective, assuming you don't live in a bubble your whole life.
This is all just my way of trying to avoid the whole "you'll understand when you're older" line that used to drive me nuts when I was a child/teenager, but well...I guess the fact that I'm ultimately resorting to it here proves that I learned from the wisdom of my elders about that. They were right, I was not...most everybody realizes this eventually.