What was the primary argument made in favor of lowering the voting age to 18

The main argument for giving the vote to 18 year olds was that if an 18 year old was old enough to be drafted into the military and die for his country, then he was old enough to vote.

Another argument for lowering the voting age can be summarised by the following quote:

"No taxation without representation"

In the US you can legally leave school at 16 years of age, start work and therefore pay tax. As such it would not be unreasonable to expect to have some say in how your tax money was spent. Therefore it could be argued that if you are old enough to fund government, you are old enough to chose who should run it and how your tax money is spent and hence should be allowed to vote.