If it is original Septic tank, it may be at end of life

"The average lifespan of a septic tank is 15 to 40 years, but it can last longer depending on how well it's maintained. A septic tank made of concrete may last 50 years or more."

Check if the drain field has large trees close. The tree roots tend to grow toward the drain field and could slow down the drainage of waters and tehre is risk of clogging and flood (dirty water flood much tougher to handle than clean water flood). 

Another thing you can ask is if there is sewer access point you could connect in the future. I converted one of my rental from ceptic tank to city sewer. You can search posting.  Had a sewage backflow event (hassle, $, insurance premium increase...), Had caustic treatment but it did not work

My primary house is also on septic tank but the whole community is on septic tank. One neighbor had a new septic tank built before selling his house. There are stuff you can flood down the drain. tank pump a few years ($500?). 

 You need septic tank pumping company to inspect the tank. You can ask the contact for their septic tank pumping company. if the frequency went up, it might point to risk.