timeline error

Rating: Null/10 (4 October 2003)

how could John connor be 13 years of age when he was attacked by the t-1000 if he was born at best, in late 1984 and if his nother fought her disease for 3 years before dying in 1997? not that I wanna be a geek about it but how could script writers, filmaker etc. working for a huge multi-million bussiness forget such a detail?