Realistically.... well what we have right now is a mess.
Age of consent in some states is 16... I think 14 in at least one state.
Drinking age almost everywhere is 21.
So, you can have sex at 16, vote or join the military or get married at 18, drink at 21.
BUT the odds are most young people are still somewhat dependent on their parents until they are 21 or 22... in many cases the parents are paying some or all of their college costs. BUT then you have others that are paying their own way, or went straight to work, or joined the military, or got married, or whatever.
I don't think raising the age of majority is the way to go. If you make it 21, then the parent is still responsible for the upkeep and behavior of a person who might be in the Marines, or working full time, or married with children.... wtf?? OR, a slacker laying around at home refusing to do anything...
Lowering it isn't much of an option. I think lowering it would cause the dropout rate to rise, which would cause the number of people in poverty and/or on Welfare to rise, increasing the burden on society.
Either leave it alone, or let "majority" be dependent on fulfilling certain conditions indicating that one is mature enough to live one's own life independently.