does IQ remain relatively constant over the years ?

Answer 1

yes, unless there is somatic damage your IQ stays the same. your brain does however deteriorate with old age. it is reflected in your ability to process data, memory, reasoning, perception, learning ability. With old age, you dont necesarily become dumber, but you could say your brain is getting used up.

Answer 2

Yes, unless there is some major change to the person in question. Of course, physical injuries or diseases (including things like Alzheimer's) can drastically effect a person's intelligence. Memory and physical endurance in things like reading, writing, and speaking will all suffer with time, too.