The graph below shows the participation rate and employment-population ratio for those 16 to 19 years old.
The graph is Not Seasonally Adjusted (NSA), to show the seasonal hiring of teenagers during the summer.
A few observations:
1) Although teen employment has recovered some since the great recession, overall teen employment has been trending down. This is probably because more people are staying in school (a long term positive for the economy).
2) A smaller percentage of teenagers are seeking summer employment. The seasonal spikes are smaller than in previous decades. So a smaller percentage of teenagers are joining the labor force during the summer as compared to previous years. This could be because of fewer employment opportunities, or because teenagers are pursuing other activities during the summer.
3) The decline in teenager participation is one of the reasons the overall participation rate has declined (of course, the retiring baby boomers is the main reason the overall participation rate is generally declining).