I deal with these heaters quite frequently. First, make sure that the circulation system is in good working order. A system that is as old as you mention may be a little "tired," not moving the water as it used to. The first Laars heater school I went to in 1991 taught that 90% of all service calls related to heaters have nothing to do with the heater. I have found that to be true. These are large gas appliances that require the proper amount of water running through them so that they literally do not burn up or start a fire. It might be time to replace your pump or filter.

When the heater lights, does it seem to "explode," a larger than usual sound when it lights? If so, the problem is most likely spider nests in the orifices. Over half my service calls on heaters that do not light are related to this if the pool circulation system is in proper order (clean filter, no clog in the pump basket or skimmer, etc.). Did two last week; spiders. You should fire your heater at least once a month to help stop this problem (once a week is better, 5 minutes is all).

Remember, you are dealing with an appliance that has some electronic parts inside that have wiring running to them. Corrosion at the terminals is often a problem. Turn all power to the pool OFF. Open the cabinet. Inside in the upper left you will find the ignition device. Remove and then replace each wire, one at a time. Turn power back ON and try again. That may also solve the problem. Did that once last week.

Beyond that, most people are just glad that their heater this old fires at all.