"Raising the legal age to 21 would have prevented Nikolas Cruz from personally buying a gun. If assault weapons were banned period, i am doubtful he would have access to a weapons black market (as a mentally deranged high schooler)."

You could really expand your rebuttal to this point even further. The proposition that the legal age should be 21 excludes all adults between the ages of 18-21 the right to self preservation. What about a single 19 year old mother that lives in a lower income part of town, should she not have access to a firearm to preserve her life and that of her child? Why is her right to self preservation okay to be infringed upon because somebody else had hate in their heart and committed a heinous crime.

The proposition to increase the age for purchasing a firearm beyond the age that constitutes legal adult status is not only bad because it wouldn't be effective in practice, it's unconstitutional and would adversely impact many law abiding citizens.