this one and this one) show that certain capabilities indeed decline from a certain age but other capabilities peak at a higher age and some do not peak at all (they can continue to grow stronger). Harvard researcher Laura Germine summarize how cognitive capabilities peak at different ages in this picture:
Aside from the domain of cognitive capabilities their are all sorts of different aspects of our functioning which may continue to grow as we grow old. For example, old age is often associated with wisdom and mildness. This may have to do with the following. My hypothesis is that as we grow old be may get better at transcending our own ego-centrism which may make us milder and more empathetic and see things in a broader and more realistic perspective. At a young age we can worry a lot about what other people may think about us. This may have to do with the phenomenon that many young people feel that other people think a lot about them and have all kinds of judgments about them. Many young people, in a sense, feel as though they are a the center of the world, not only to themselves but also to other people. While they think that other people think a lot about them, this is not the case. Other people are primarily thinking a lot about themselves and their own lives. Eleanor Roosevelt once summarized this as follows: “You wouldn't worry so much about what others think of you if you would realize that they seldom do.”
As we grow older we may develop an ever more realistic perspective. Perhaps this more realistic perspective creates new opportunities for progress. That more realistic perspective may be - I am saying 'may be' because I am speculating; I am not yet that old - that other people are complex just as we are, and that they may have doubts and worries just like us. We may realize that we are less unique than we thought and that we may think less dramatically about our own discomforts and suffering. If we do, and thus transcendent our own ego-centrism to some extend, we may make free more room for focusing on helping other people and for focusing on improving our living environments.