30 year old septic tank - never been pumped.
I built this old house in late 1980. At the time, suburban Georgia didn't have sewers so we went with a normal septic tank. They put sewers in about ten years ago and said we couldn't maintain the septic system and would have to hook up to the county sewer when the tank needed service. Well, still not hooked up.
Secret? Maybe. We have put Rid-X in once a month but I really believe that installing removable screen traps in all the drains and the wash machine hose has prevented pounds and pounds of kitchen and laundry debris from filling up the tank. When the screens clog up we just take them outside and wash them in a bucket of water and replace them.
Who knows? Maybe the septic tank will outlast all of us.