Teenagers suffer the same as oldies but because their brains are still wiring whereas us oldies brains are unwiring, hence teenagers get 'better', oldies don't. Read that in a study on brain disease a few years ago.
Old age may not be as much as factor as prior drug use (legal and illegal). I've noticed by middle age a pot-head / prior pot user or alchie has lost a lot of cells and is often struggling with basic mental function rather like early onset alzheimers. It's like they are trying hard to comprehend but the circuits aren't fully connected. The great majority of older folk I know who haven't destroyed themselves thus are plenty sharp.
Anecdotal evidence! I was told by someone else who heard someone else talking about something they read! Perhaps the use of longitudinal studies, whereby a person's progress or regress is tested say every 5 years. This would be assuming that those that participate are subject to the same environmental conditions, etc as each other. A good example of longitudinal studies was that conducted in Broken Hill for the effects of lead dust. But then what are you actually testing for? Are we talking use of control groups, randomised trials perhaps. Is everyone's brains wired the same way? Do they slightly differ in the shape of the lobes, ventricles, etc? Who was subjected to events that affected their neuroplasticity at an early age. Maybe the answer is that everyone in the whole wide world undergoes a full MRI scan of the brain (nah full body) at age 20 say, in the event that in their lifetime they may want to learn to fly as a recreational pass time.
As for teenagers and their brains still wiring, it is a known that the connection between the frontal lobes particularly in males does not occur until their late 20s. This leads to impulsive behaviour. Drug and alcohol use - every day I see it through work where people young and old have fried their brain cells and they then have the auditory and visual hallucinations for the remainder of their lives. They object to the use of psychiatric drugs claiming that its not a natural product like the THC or the alcohol that they used and the world takes care of them forever more. Its really sad to have to do a modified mini mental state examination on a 35 year old because you know that the person concerned hasn't a hope in hell of answering all the questions and then they score 7 out of 15. Yet the young ones never think this will be them in a few years if they don't stop now.