For what age is this game appropriate?

Approximately 12 years old and above. Before this age, it is still possible to play and learn the 10 fingers method, but to really enjoy and have fun with it, a minimum of typing experience is a good asset. It is possible that younger kids, especially those with musical experience could be good at this game.

If I already type with the 10 fingers method, can this game help me to write faster?

Absolutely. This game focuses especially on the mechanical aspects of typing. Its core mechanic relies on the methodical training of professional musicians, on the exercises that they do to become virtuosos. Typing keyboard is a similar motor task as playing the piano and the same methods will lead your hands to smooth and agile movements. Of course, you need to be constant with your practice and it's better to spread your training sessions over a constant period of time and avoid doing too much at a time.

If I have a QWERTY or QWERTZ keyboard, which is not the US QWERTY or the German QWERTZ, can I still play the game?

It depends. If your keyboard layout (you can see it in the Windows settings of your computer) is in the following list, you will be able to play with letters and custom text that you enter, but some special characters might not be available and others at different places on your keyboard than they appear in the game.
All the supported keyboard variations QWERTY and QWERTZ are highly similar to the US QWERTY or the German QWERTZ, so you can still have a lot of fun and improve your typing skills with this game.
List of supported keyboards:
Optionally, you can change your keyboard layout to US QWERTY or German QWERTZ, the game will detect the new keyboard and you will be able to play with your keyboard and that new layout. The US QWERTY layout is good for programming.

How often and how long should I play to see improvements with my typing skills?

Just like learning a musical instrument requires constant practice. The purpose of this game is to help you train efficiently in a way that you can improve your rhythmic and typing/coding skills, as well as your concentration.
The rounds are 20 minutes, not less, not more. At the end of each round, you can see statistics with your performance and your are encouraged to close your eyes 2 minutes to rest and recover (performance training is more efficient with pauses).
When learning a musical instrument, you can start with 20-30 minutes a day and if you progress, it is up to you to choose how invested you want to train, but 20 minutes per day or every second day playing this game is a really good start. If you want and you feel your fingers are fit, you can increase the time as you wish, but constancy is more efficient for training than playing longer and less often.

Can I get eye dizziness when I play? What can I do to avoid that?

Yes, you should avoid staring at the characters when reading them, especially at higher tempo, since they always travel in the same direction, from top to bottom of the screen, like many other rhythm games. It is possible to sweep gently with the eyes across the field and avoid staring and ideally, when you have learned which finger presses which characters, you can look only in the upper grid and read normally, even at high tempo. If your eyes are strained, you should stop playing until your eyes are alright and perhaps play for shorter time periods at a time. Your eyes are more important than completing a 20 minutes round!

If I play just this game to train my typing skills, how good is this?

Good, but not optimal. This game helps learning the fingering of the 10 fingers method, and bring virtuosity (precision, relaxation and speed) to the fingers through the rhythmic training. I think it is best to use this game and also train your typing in real-life situation. There are also typing training software that have a different emphasis as this game and they may be a good combination to the method used in this game and the musical flow it gives you!

Can I become better playing a music instrument if I play this game?

Yes! In this game, will train with 39 different rhythmic patterns and most of them are used by musicians when learning songs or practicing scales and arpeggio as examples. Playing this games, you will get an increasingly good sense of rhythmic timing and you will be able to use the 39 patterns and your acquired experience for musical training, especially for metronome training and playing songs that have those rhythms in them.