The "Madagascar 3" star, who raises Trey, 19, Jaden, 14, and 11-year-old Willow with husband Will Smith, said she grew up in such a tough neighbourhood that she never expected to live so long, reported Contactmusic.
"I grew up in a drug-infested neighbourhood where you walk out each day and you just hope that you make it. I came from a war zone.
"There was a possibility that I wouldn't make it past 21 - that was the reality. When I turned 40 it was a surreal moment because I had never imagined reaching 40. I looked at my husband and said, 'Oh my God, I'm 40! What am I going to do now?' So yes, it does put things in perspective'," she said.
Jada, who was raised by her drug-addicted mother Adrienne and grandmother Marion, said she is thankful that her own children do not have the upbringing she had.
"I'd be terrified if I had to raise my kids in the environment I grew up in. When I look at them, I see them flourishing at such young ages."