If you're interested in the subject there are several references in the wikipedia article and here ( http://www.newscientist.com/article.ns?id=dn4918 )

There's something about music that can can harmonize the brain. Getting logic and imagination to concentrate on the same thing from different angles. Mozart was a genius as shown by the way his music can arouse emotion and make the listener feel part of it.

Will it work for 20 something year olders too?

I believe the fetus heard and feels vibrations in the womb and that those vibrations are distinguished as pleasant and not pleasant. Music is mostly relaxing and calming to the psyche. Perhaps these tones promote development of in the regions of the brain that deal with retention. Thus allowing the child to learn faster.

I believe the fetus heard and feels vibrations in the womb and that those vibrations are distinguished as pleasant and not pleasant. Music is mostly relaxing and calming to the psyche. Perhaps these tones promote development of in the regions of the brain that deal with retention. Thus allowing the child to learn faster.

is it only his music or other composer's will do as well?

Bach as well, I think, hippo...- baroque music!