You can find the information on her wikipedia page:
Very little is known about Sarah's life prior to May 12, 1984. The
Terminator does not specify her age or birth date, although according
to the original script (available on the Special Edition DVD), she was
19 years old. The film was primarily set on May 12–14, 1984; according
to the script, her birth date would be between May 15, 1964, and May
11, 1965.
In Terminator 2: Judgment Day, Sarah's psychologist states she is 29.
The film takes place when John (born February 28, 1985) is 10, placing
Sarah's birth date between March 1, 1965, and February 25, 1967,
making her either 17, 18, or 19 during The Terminator. Many of the
franchise's hardcore fans and writers of "Terminator" novels have
speculated Sarah's exact date of birth is November 13, 1965, because
this date is compatible with her given age in The Terminator and her
stated age by the end of Terminator: The Sarah Connor Chronicles in
the final episode "Born to Run".
The tombstone shown in Terminator 3 reads 1959–1997. This birth year
would make her 24 or 25 during The Terminator. She and John were
living "off the grid" in her adult years, routinely using false names,
birthdates, inter alia. Her tomb was really a weapons cache. She dies
of leukemia sometime after "Judgment Day" (August 29, 1997).
Terminator 3 changes John's age in the second film from 10 to 13,
introducing contradictions in the films' datings that cannot be
reconciled to one timeline.
Linda Hamilton was 27 when Terminator was made.
In terminator Genisys it also stated that Sarah is around 20 in 1984 (She mentions that she was 9 1973, when Pops saved here)