The first graph conveys a much better understanding of the information though, because of the way that the information is organized, grouped, and presented.
In the first graph, the data is grouped by the form of usage rather than the teens age like it is in the second graph. This allows the viewer to compare between forms of usage for one particular age group, but also across multiple age groups.
The first graph also presents the total percentage across all teens, which helps the viewer see the information in a larger context.
Both graphs also use color coding to differentiate between the bars, but the color coding is more effective in graph one because in graph two the viewer has to constantly check the key to verify which form of usage the bar is referring to, where as in graph one the viewer will always know that the darker the shade of blue, the higher the age.