Despite being friends for 18 years, the actors have been romantically involved for only the past eight months and “they want to get married very soon” according to the source.
“Charlize has seen Sean grow up,” said the source.
“He’s a wiser soul and much more patient,
“They’re looking to the future with a lot of hope.”
The 38-year-old Benoni beauty and 53-year-old Penn are reportedly wanting to adopt a child together upon tying the knot.
Theron already has a two-year-old son, Jackson, and Penn a 23-year-old daughter, Dylan, and 20-year-old son, Hopper, with his ex-wife Robin Wright.
According to reports, Penn is already building a relationship with Jackson.
The insider described Penn as a father figure to Theron’s son.
The source added: “Charlize always intended to have another child.”
Penn will be directing his film, The last Face, starring Theron, in South Africa, this summer.
Penn was reportedly spotted on arrival in Cape Town, on Tuesday (July 15).
The film is due to be released next year.
The insider said Theron and Penn have watched each other change over the years and are looking forward to a positive future together as a couple.