Re: long term effects of Concerta?
As I understand it, Concerta (or any methylphenidate medication) can delay physical development, but does not have an effect on one's ultimate physical size. I cannot speak to how this was determined, but I can tell you that methylphenidate has been in use to treat ADHD and other disorders in children since at least the 1950s, and has been extensively studied over very long periods of time.
Human growth does tend to stop at around 20 years old, though there are often exceptions, especially in very tall families. It's not unusual for men to continue to grow in their early 20s, so don't lose hope yet. Ensure that your diet is nutritious so that lack of building materials doesn't limit your potential growth as well.
As far as mental development, I'm not aware of any research that indicates that methylphenidate has any negative effect. I certainly would be prepared to believe that it does, however. That said, the brain is an amazing organ that grows and develops as it is needed, much like a muscle (it's now believed that lifelong learning is protective against degenerative brain conditions like dementia). Make sure that you continue to work and learn, and any lack of brain development should right itself. Unless you have been abusing your Concerta in a rather extreme way (too much methylphenidate at once has neurotoxic effects), I wouldn't lose sleep over that one.