Some general advice is below. While we've found them lasting longer, a common life expectancy of buried oil tanks is 10-15 years. We do not have the same data for gas tanks. Life may be similar. Buried tanks should be tested for amount of water present in tank bottom, and water should be pumped out.
Water corrodes the tank and leads to leaks.
Leaks can also be due to damage at time of installation, improper installation, corrosive soils, or piping defects. If the tank is to remain in use, ask your fuel supplier about using an additive or other methods to help remove water. They also found 2.5 fuel line leaks per 1000 customers. We do not have similar data for buried gas tanks in residential application. Westinghouse Hanford to participate in an interesting non-residential study.