Ask a psychologist. Any psychologist, actually. The one at our national Mensa could give you some hints, but I'm afraid a meeting with him is hardly possible.
Intelligence increases for some time, then decreases after some time. With hard practice, you can slow down the setback, but that's pretty much everything you can do.
Age is incorporated in IQ tests (they are standardized considering the test takers age), and for them to be accurate, they are balanced according to age. For example if you reach a 75/100 score on a test and you're 18, your IQ might be around 125. If you score 75/100 with the age of 40, your IQ is around 100.