story identification - Book ID: Boy plays a video game and has to to solve different tasks (the last being a maze that's an IQ test)

There is this boy who plays a video game where he has to solve a multitude of tasks. One was a tactical space battle and the last one was a maze.

Later, it's revealed that it was some IQ test and he ends up in a facility with some other kids. In the facility there are brain-washed adults that care for them. And they are supposed to solve some kind of problem for some ominous evil figure.

Also, the boy has a friend who, after the boy disappeared, also barely manages to beat the game to get to see his friend again.

Answer

Brainboy and the DeathMaster by Tor Seidler

Darryl Kirby is an orphan. He is also a whiz at computer games -- especially those conceived by Keith Masterly, the greatest software guru of all time. But when Darryl plays a new prototype of his favorite game, StarMaster, strange things begin to happen. After he finds his way through a tortuous maze and answers questions that could stump a scientist, a mysterious message -- "WANT TO PLAY?" -- invites him into a game in which the stakes are higher than they first appear -- a game that eventually lands him in a luxurious laboratory where Masterly has enlisted a brain trust of children for a secret purpose of his own. Befriended by Nina, who suspects Masterly's motives, Darryl slowly uncovers a sinister plan and finds himself the star in his own real-life computer game. But will he and Nina be able to stop Masterly and escape before it is too late?

Brainboy and the Deathmaster is a fast-paced, high-tech mystery-adventure by National Book Award finalist Tor Seidler, author of the modern-day classic Mean Margaret.

The German description found here (in German the title is "Danny und der Herr der Spiele" - "Danny and the Master of Games")

Aus dem Amerikanischen von Peter Knecht. Der zwölfjährige Danny ist ein Computergenie - da kann der gleichaltrige Boris nur staunen. Auch der reiche und geheimnisvolle Computerspezialist Keith Masterly ist von Danny fasziniert und bringt ihn in sein geheimes Labor, das "Paradise Lab". Zu Dannys Erstaunen trifft er dort sieben weitere hoch begabte Kinder, darunter auch Nina, die Schwester von Boris. Bald wird den beiden klar, dass es in dem paradiesischen Labor nicht mit rechten Dingen zugeht. Als sie entdecken, dass Masterly die Kinder nur benutzt, um die Formel der ewigen Jugend zu finden, beginnt ein Wettrennen um Leben und Tod. Gerade noch zur rechten Zeit taucht Dannys Freund B.J. auf, der ihn seit seinem Verschwinden sucht.

My own translation:

Translation from English from Peter Knecht. The twelve years old Danny is a computer genious - Boris, who is the same age, can only marvel at this. The rich and mysterious computer specialist Keith Masterly is fascinated by Danny, too, and brings him into his secret Laboratory, the "Paradise Lab". To Danny's amazement he meets seven other highly talented kids there, including Nina, the sister of Boris. Soon both will realize that some things are odd in the paradisaic lab. When they discover that Masterly just uses the kids to get the formula for eternal youth a race to the death begins. Just in time Danny's friend B.J. arrives, who was searching for him since he disappeared.

I remember that drugs were involved, though I can only distinctly remember that the kids were supposed to take drugs to be submissive and as a little enhancement for their IQ (not sure if that last part was just the promise of Keith Masterly to get them to take the drugs though). Of course the protagonists decided that it's better not to take them in order to discover the secrets of the laboratory.