You just gave us all your 'elevator pitch' here so I would just have that prepped and ready to go if you come across anyone giving you grief on your age. 

You could conceivably divide the buyer group even further such as first-time buyers, age, location, etc.

Average age of the homes?

I’m 18 and my goal is to get my first rental before I turn 20, but I’m not sure how I could go about it like if course save up money but how could I ever get a loan at such a young age to finance it?

I know some people your age who are killing it.First off if you are going to partner up make sure you have a thorough agreement in writing spelling out roles, investment, profit splits etc.You will need a title company or closing attorney to close your deals.

many in real estate sales never really learn proper technique its a learned skill.. there is a reason there are top performers in real estate.. they simply have better skills you can learn those skills.I was taught them at a young age and by some very sharp salesmen..

You should build up reserves, 5-10%, from rent to meet those expenses depending on the age of the building and condition.

Supply and demand heavily favors the solid GC in the current market; my retirement age GC father in law says that he's raised his retail first time customer rates 20% per year for the last 4 years and, in his words, "no one has even blinked."

I've only done a handful of flips but I always replaced the heating systems, roofs, and any other major system that was even remotely borderline on age or wear. 

I actually told her maybe she should get rid of the cats just so she would have more options for apartments to rent, but she's a crazy cat lady already at age 20 so I knew my advice would go in one ear and out the other!