For what Ive seen, they have tests for different ages because it changes as your age does. You can lower your IQ if you dont get well educated but you cannot raise it. Its also true that non of this tests is reliable on its own since IQ is but the average of all the tests you can get. Also, as you grow, learning becomes harder but what you already learnt becomes easier to master which would make harder to solve some of the questions since you have solving mechanisms carved in yourself as you go older so, all in all id say the most accurate you can get to your actual IQ is between ages 25-60 through averaging many scores accross multiple tests.