Nicks, 64, talked about the casting for a possible future bio-picture in her new documentary, "In Your Dreams," which followed her as she made her 2011 album of the same name.
"I've already told [Reese] she's almost too old," Nicks told "ET Canada" at the documentary's Toronto premiere this week.
"I love her, but she's like, 'I could play your mother.' I'm like 'O.K,'" Nicks said of Witherspoon.
Witherspoon won an Oscar, Golden Globe and SAG Award for her portrayal of singer June Carter, Johnny Cash's wife, in the 2005 drama "Walk the Line."