Normal, cognitive changes as you age are completely normal. This may include mild cognitive impairment, which is quite common in humans after midlife.
Usually this "dulling of the mind" starts around middle age. Since the average person lives to about 80 years old, this means that if you’re 40, you are middle age. Even if you are nowhere near 40 right now, this age comes sooner than you think because time passes very fast while you’re living it.