Our home is 20 years old. Our dining room light switch recently went out. We keep replacing the switch( 4 times now) and it's out again. Could it be the light fixture causing the problem? If not what should we do?

Replaced the switch 4 times? Sounds like Einstein's definition of insanity- just kidding.
Of course, the problem can be in the light fixture or any other points in the circuit, or even the bulb. How long does it take for the light to malfunction after a switch is replaced? What type of light fixture?
Provide as many of these type details as possible and the experts here will help you narrow down the cause of the problem.

Are you using back wire or are you wrapping the wire around the screws? You can buy a heavy duty switch, not the. 49 cent variety. How many watts are you running through the switch?