Like Gypsy said, the sentence of June 16 1996 may not relate to Emma’s crime. Also Emma could have been on the run before she was arrested so although she may have committed the crime in 1996, she may not have been arrested until a year or so later.
We know Emma was 18 when she gave birth to Henry, but she could have been 18 years and 7 months old, making her close to 19 but still 18 when she gave birth or she could have just turned 18.
If we base the show as starting in Fall 2011. 28 years ago would have Emma arriving in 1983. Add 18 years would have Henry born around late 2000/ early 2001.
Also the article was written by Sidney as a smear campaign so some details may be exaggerated.