The worst thing young adults can do is go to graduate school because they aren't sure what else to do or they can't find jobs. Grad school, after all, is often an extremely long commitment. A Ph.D., for example, can take six years.
The average lifespan for an air conditioner is 10 to 15 years. Some air conditioners last up to 20 years. As interesting as these data points may be, they don't tell you the whole story. The issue is that a variety of factors affect the lifespan of an air conditioner.
If it took them 1 hour to complete a coil change out they did not due a proper pulling of vacuum on the line sets and left moisture and contaminants in the lines. This process usually takes 45 minutes to 1 hour at the minimum after the coil is installed and the line sets are sweated in.