An industry survey found that heat pumps built in the 70's and 80's lasted about 14 years if regular maintenance was done on them. There are some other things to consider, though. First, if a major component, like the compressor goes bad on any unit over 10 years old, the replacement costs are a lot more than it is worth.
Second, A new unit will be almost twice as efficient as the older one and that will literally pay for a new unit in just a few years. Plus a new unit gets a new warranty.
On all new Maytag units, there is a 12 year warranty on all parts. So, is it time to replace your heat pump? It might be, especially since there may be tax credits available. You'd be letting the government pay for a portion of your new heating and cooling system.