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The gas valve is not opening to allow for the main burner(s) to ignite.
Thermostat, low voltage wiring, transformer, and the gas valve are all common problems that result in what you discribe.
A less common but possible problem could be a gas regulator or the gas pressure itself.
There isn't room here to explain all of the hows & whys, but before you start trial and error replacement, it might just be cheaper to call in a expert.
An expert can also make sure that the thing is operating safe. Carbon monxide leaks can and do occure with age.
At 24 years of age, I wouldn't spend a whole lot of money on it. Ask the utility company to do a return on inverstment analysis for you. If a new furnace will substantually cut your heating bill, that may indeed be your best choice.[?]
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