The estimate of Jane's DOB comes from Laurence's estimation that they are approximately the same age. See Talk:William_Laurence for information on Laurence's age.
In HMD, Laurence estimates that Jane is his age or a bit older. However, in EOI, he refers to her as being definitely older than he is - it's one of the reasons he's hesitant to introduce her to his mother (which happens anyway in VOE). Rose 15:35, 4 December 2008 (PST)