How to create a discussion about the "age of majority"?

I want the U.S. to make it 21 in every state but 18 for voting and fighting in war but everything else 21.In the early 1970's some states enacted the "age of majority act" which was to lower the age of majority from 21,in most states its 18 2 states 19 and Mississippi has it 21 still.How can I bring up a discussion like a bill to require all states to raise it to 21.At 18-20 you are in college and you need time to mature and 21 is perfect since the brain matures in the mid 20's.