This complex question has a somewhat complex answer.

Babies are born with the potential to develop brain connections (synapses). It is now believed that the number of potential brain connections is present at birth, but the potential for connections is just that - potential. The number of potential connections that could eventually be developed are believed to present at birth. A comparison between the newborn brain and a telephone book can be made. If you imagine a phone book that has no specific numbers next to the names but has the names, which means it already has the potential number of entries established, you get some idea of how undeveloped, potential, brain connections are present at birth.

When those connections are developed in the early years of life, it is like filling in the telephone numbers of all those people listed in the phone book.

A very large part of brain development occurs in the first three years of life. When potential connections aren't formed brain cells die. Even after the first three years of life, though, the brain continues to mature. It is now believed that the last part of the brain to mature is the prefrontal cortex, which can take until the early- to mid- twenties to finish growth. It is believed that teenagers use a different part of the brain for some of the thinking processes, while the prefrontal cortex is in the process of maturing. A PBS special on the brain explained that because of the immature prefrontal cortex teenagers may be prone to depression and/or misinterpreting things like the meaning of some verbal or non-verbal communication.

Since bone development is known to be complete at around 25 years of age, it makes sense that the brain's finishing touches would be complete at around the same time.

Separate from the matter of the physiological growth and maturation of the brain is learning. As we get older we learn. We take in more information. We store what we have taken in, and we generally have a "larger collection of files" from which to draw conclusions, and on which to build yet more knowledge. On the one hand, IQ is said to be fairly fixed early in life. On the other hand, the more information a person has taken in (through learning and living), the better a foundation he has for building on that learning.

Another factor involves the individual differences between people, when it comes to specific cognitive skills. The person with particular maturity or "advanced thinking" in the type of intellectual processing involved with human nature, knowing oneself, and understanding other people may feel older than the person who has immaturity in this area of thinking.

How young or old someone feels can also involve their circumstances. The person who has been through more tragedy than most others his age have may feel older in some ways. At the same time, the same person (if his attitude is positive) may be very "young at heart" in other ways. Teenagers who have used drugs or alcohol frequently can actually slow down the maturation process that comes with experiencing life as a clear-headed teenager. Many people who use drugs and alcohol in their teenagers get to be in their early twenties with a general emotional maturity level of someone much younger.

People who are victims of psychological or emotional abuse can feel very small as a result of feeling helpless for too long. Feeling small doesn't need to include feeling young or being immature, but it can cause lack of the kind of confidence many adults have.

Having discussed all that, there is one very nice thing about growing up and getting older: Under normal circumstances (and as long as no disease is present), people feel very much the same when they're older as they did when they were very young. My late mother, suffering from long-term illness and lack of mobility at 76, used to say, "What people don't realize about being older is that you can't do the things you used to be able to do, and that you want to do, but you still feel exactly the same as you always did on the inside."

I'm nowhere near my 70's, but I'm old enough to have my youngest child attending college. What I've discovered - as I guess most people do - is that my mother was right. I'm a good, solid, mature, age at this point; but I still feel exactly the same as I did when I was very young - only I know more and have more confidence in that person I am on the inside.

I guess my answer to your question is this: There is physiological brain growth that, even if the potential to develop some new connections and new brain cells exists, reaches a point of general maturity. There is emotional maturity (which is, to the best of my understanding, related to the development of certain parts of the brain). The is IQ (the measurement of ability to process information efficiently), and IQ is a composite score after a certain set of cognitive skills have been tested. There are also psychological factors that aren't necessarily associated with the brain's physiology, and that can be affected by things such as grief, stress, illness, and general experiences.

Maybe most important, though, is the "young-at-heart" factor. The most fortunate of people are those who are healthy and mature but who manage to keep just a little bit of that "young-at-heart" factor that can help make life a little more positive, help keep them healthier (the mind/body connection), and, most importantly, help keep them connected to the person they've always been.

I suppose I could have simply said that there's a point where we grow up and stop feeling older and older with each year. (Ironic, isn't it?)