Japanese manga demographic evolution

The evolution of Japanese Shōnen and Shōjo demographics is highly interesting! 

Before Shōnen Jump got launched in 1968, the audience generally consisted of what are now called preteen boys. However, after Shōnen Jump’s launch, the age demographic possibly went up from mostly preteens to mostly teens. 

Before Shōnen Jump - 9-12 year olds

After Shōnen Jump - 12-18 year olds 

Same for Shōjo, albeit until Ribbon got launched in 1955, whereas its sister publication Margaret got launched in 1963, making the formerly specialised audience broader as well.  

Before Ribbon and Margaret - 9-12 year olds 

After Ribbon and Margaret - 7-15 year olds