Observation and research indicates that even with age there is no change in a person's ability to think and reason. Actually, you may be able to draw more meaning from a given situation and verbalize it better with the benefit of a few years of experience. Furthermore, there is evidence to indicate that in the absence of an illness, intelligence levels remain stable. Adult learning abilities do not decrease generally, but the proficiency of learning some types of things is reduced. That is why it is easier for a child to learn a foreign language than for most adults. It also seems that the 'use it or lose it' theory that is often associated with physical activity can also be applied to mental functioning. Apparently, the more stimulated the brain is, the more it will continue to work. Educated and mentally active people show little decline in test performances. In fact, accuracy often improves with mental activity to a considerable extent.
Memory is not altered in quite the way that myth would have you believe. Primary memory or recent memory, appears to remain stable with increasing age. Secondary memory requires that the primary memory be transferred into the storage system so that it can be retrieved at some later stage. Here the findings are mixed, but there does seem to be a slight decline. This secondary memory retrieval works better if the information to be retrieved is meaningful. This indicates that you do not lose your memory as you grow older. There is a correlation, however, between depression and poor memory.
To summarize, it appears that people who perform well when they are young will continue to do so as they age. The changes that do occur do not happen until the late seventies unless there is some chronic illness or sensory deprivation. Even then, you can compensate for these changes by using aids such as glasses or hearing aids, using caution, and giving yourself more time to accomplish interest-oriented tasks.