Favre's comments came Thursday during a radio interview reported on "CBS This Morning."
"I don't remember my daughter playing soccer, youth soccer, one summer. I don't remember that," Favre said.
Favre, 44, played in a record 297 regular season games in a row during his career, during which he was sacked 525 times, suffering an unknown number of concussions.
"When I first started playing, those first 10 years, they didn't keep a log like they do now, so there's no telling," Favre said. "I think after 20 years, God only knows the toll that will be taken as time goes by."
Dr. Robert Glatter, a former New York Jets sideline physician, said Favre's memory loss and other symptoms are typical for football players.
"Often the CT scans or the initial scans we do after a suspected head injury, which is severe, may be normal, but there's functional changes -- changes involving memory," Glatter said.
Earlier this year, the NFL settled a concussion lawsuit with 4,500 retired players for $765 million.
"What we were able to do was all compromise in our admissions. There was no admission of guilt, there was no recognition that anything was caused by football, but the reality is we want to help our players," NFL commissioner Roger Goodell said in September.