My house was built nearly 200 years ago, but the current septic system is from the 1950s. When I bought the place in 1985 I was told to have it pumped every 5 years. I had it pumped in 1990 and the guy said I could probably wait 10 the next time, since there are only two of us. I had it pumped again in 2000, and was told not to bother again until 2015. That's the current plan.
However, since it only costs $99 to have it pumped it's certainly not prohibitively expensive to have it done every 10 years or so.