inexpensive car for teen

Both the Aura or the Altima are good choices. The Aura is the same car as the Chevy Malibu but can be purchased cheaper because GM discontinued Saturn in 2009. Also, look at the Ford Fusion, Toyota Camry, Mazda 6, and Honda Accord. Good used examples of all of these can be found in your price range.

You didn't give us much to go on but as you suggested midsize I assume that is what your after. If smaller is an option you could look at a gen 3 Prius that are getting below the $6k mark.

Buying cars from dead brands is a good strategy for getting band for the buck. Pontiac, Saturn, Saab, Mercury are all worth looking at

There are many negative reports on the internet which caution against Honda Accord and Civic due to automatic tansmission failure.

Toyota Camry and Corolla appear to be the best buy in the $2000 to $5000 range.

For $6,000 and under 120,000 miles I'd look at a 2012 Chevrolet Cruze($5,000)as it is the newest car you can buy within your criteria. It'll have the most airbags and safety features compared to decade old, Takata airbaged Civics or Corolla.