How can I make a case for this for the Government?

I want the U.S. To change the "Legal Age" or age of majority meaning age you are no longer considered a minor to 21.You are already considered a minor for drinking and I even heard they are thinking about raising the smoking age age to be a star and they even raised to age to get credit cards to 21.Why dont we just make it easier in ALL states to make you an Adult at 21.I have a cousin who was 19 and he went to this party and these men in their 20's were calling him a child and they were saying "take your little azz home boy" and he got offended because they didn't see him as an Adult.I want congress to p a bill or something to get ALL states to raise their majority ages to 21 with Obama signing it off.In Alabama you become an Adult at 19 in Nebraska 19 and Mississippi its 21.Mississippi is the only state that has it right.Society is like unless you are the legal drinking age you aren't an Adult.I am sick of these 18 plus or 21 plus either 18 or 21.