The laws have always been a bit confusing, but as the other posters have stated, you have to be 21+ to buy from an FFL, and 18+ to buy private sale. I am a Christian, so following the law is extremely important to me which is why I searched extensively to find out if I could buy a handgun at 18 or not after seeing that anyone under the age of 18 is prohibited from hunting with a handgun posted in TWRA's hunting guide magazine. The TN department of homeland security also has a exception to the carry age of 21 for 18+'s who are active duty military/honorable discharged personnel.
I bought my first handgun at 18, and I've since owned nearly a dozen handguns bought through private sale, and spoken to numerous police officers, including one that pulled me over since I accidently pulled out in front of him(low blood sugar!) and he said I was completely cool to keep a handgun in my car.
While I will be turning 21 April 6th, I am a strong supporter of the right for this age group to be able to purchase handguns private sale, and wish that both more people were aware of the laws and that this group could buy from FFLs.