Good septic systems will last 50 years or longer. The longevity of a system will be determined by the quality of the soil, the maintenance of the system and its usage. Whenever I am getting ready to list a 
home over 20 years old I insist that the homeowner get a complete inspection. This is not the same as having the tank pumped out or even the inspection by the town where the sanitarian walks the property every four to five years to make sure that sewage in not breaking ground. A thorough septic inspection will cost approximately $400 and is done by a septic company.
They dig, by hand, the intake and outtake areas of the tank. They check the baffles to make sure they are intact. The baffle prevents the solids from flowing out into the fields. The inspector can also judge the highest level that solids have been in the tank. Then, again by hand, they locate and uncover the distribution boxes. A D-box is located at the junction of the main pipe going into the yard and each intersection with a field. There are usually 2-4 filds so 2-4 D-Boxes. These can deteriorate over time. The inspector will run water to watch it flow out of the tank- through the D-boxes and into the fields. He can judge the approximate useable life left in the system by where the water goes. If all of the water is going into the first field and none into any of the other fields, it indicated a long expected life. If the water bypasses all of the fields and goes slowly into the last field, the septic is nearing the end of its life. It is not failed but a Buyer will ask for a replacement or credit for a new septic system.
Replacing an entire system can run $25,000+. This is not a surprise you want to have after you have fixed up the house, marketed it, negotiated an offer and started packing. You want to know this up front. A system might be fine but a component needs replacing. A lid on a D-Box or the tank might run $100. To replace the D-box might run $400. To replace a baffle, a couple of hundred dollars and to replace a cracked septic tank about $5,000. This is the most expensive system in your home. To be able to market that the septic was pre inspected is a huge selling bonus when you put it on the market and can put money in your pocket. Many installer will work with you so that you pay a portion when work begins and the remainder when the home sale closes.