Simplify the following text for a Grade 6 native English-speaking person.
Original: This is perhaps one reason why it is difficult to find points of similarity between the music of Johann Sebastian Bach and that of Johann Christian .
Simplified: His father was already 50 when J. C. Bach was born , so it is not surprising that by the time Johann Christian had grown up the style of music had changed a lot since Johann Sebastian had been writing his great works . Johann Christian belonged to the Classical period .