I made a quick research study over the average bowling scores by age and here it is:

The average bowling score for a beginner is around 20-70 points. With practice, most teens and young adults average between 120-190 points. Scores on the higher end of the range take considerable practice, but they are certainly achievable. Average bowling score by age for kids under 10 is usually around 20-70. Unfortunately, low bowling scores can be disappointing for kids, especially those older than ten. Average scores for 10-12 year olds are around 70-120. Kids this age may rarely get strikes, and spares might be few and far between. Average scores for 13-19 year olds with practice is around 120-190. Adults aged 20-30 score an average around 170-220. Those aged 30 years old and over might average around 200-250 with years of practice. The maximum score in bowling is 300, which can be achieved at any age. But then again, what matters is the amount of practice and the time put in.