According to this Article The commonly held belief that, with age, our memory and cognitive function begin to decrease substantially may be untrue. Many of us have the notion that, once humans go "over the hill" and reach the ripe old ages around their Sixties, they will show a sharp decrease is the ability to remember facts and daily routines of their life. This is known as the start of being "senile", but this research suggests that the start of this process occurs much earlier than most of us think.
The Study of about 350 people between ages 20 and 90 shows that cognitive function starts to decline as early as mid-twenties. The gradual decrease is just so faint that it is hardly noticeable until it affects us enough to disrupt daily activities when we are older.
I found this very interesting as sometimes, even now, I find myself taking longer to learn new things than I ever used to.