Aunt Edna's Vote

"Who did you vote for?" Helen asked her friend Patty.
"Obama," Patty responded.
"Good. That's who I voted for."
"You're living with your Aunt Edna, right?" Helen continued.
"Yes."
"Do you know who she voted for?" Helen prodded.
"She didn't vote."
"Why not?"
"They wouldn't let her register to vote."
"But we live in a democracy. It's everyone's responsibility to vote. She's a citizen, isn't she?"
"Yes, she is a citizen."
"She isn't a felon, is she?"
"No, she isn't a felon."
"Then why won't they let her register and vote?"
Can you think of any reason why Aunt Edna might not be allowed to vote?