It is recommended that you take the test as early as possible. This is because the younger you are, the less likely it is that the diseases you are susceptible to have already developed, and there are more opportunities for preventive measures to be taken.
Additionally, it is more effective if you try to reduce the exposure to the other causes (enviroment factors) for the diseases from a younger age. The older you get, the more likely it is that the damage has already been done somehow by repeated exposure to certain elements which may contribute to the onset of the disease.
In any case, even if you have already contracted a condition, taking the test is still recommended as you can still work on reducing your risk for other diseases so that your condition will not be aggravated.
Therefore, the best time to take the test is NOW!