Changing line-width in a line chart with a lot of series

I am making a line chart with a lot of series (age categories 0-4, 5-10, 11-15 etc etc). My chart comes out fine but due to the number of series the chart gets almost unreadable. Therefore I would like to change the width of the lines (from 2.25 to 1). Is there an option to change all the lines at once, because I am only able to select 1 line at the time and this would take a lot of time?

I am using excel 2010

Thanks in advance