As per my experience i would like to share that AC's have no particular or specified age limit. It is just that when your AC stops cooling or perform well and starts giving you trouble than it should be changed. Many people get them changed when they experience low/no cooling, gas leakage, blower not working proper etc. But usually an AC can run for say about 5 years or more.
While replacing AC you should take care of many things as the AC available these days are more advance and also energy saver if you pick the right one. Some of the things are warranty period, company brand, energy consumption, level of energy star (the more the stars the more energy saver the AC is.
There are many appliance store that can help you
buy home appliances like Air Condition but you have to select the best, by doing proper research you can save a lot on them.