What you're calling "statutory rape" California calls unlawful intercourse with a minor 3 years younger (Penal Code 261.5(c)). The statute of limitations on that charge is 3 years from the incident, not three years from the DNA test. If I understood your fact pattern correctly, that charge cannot be filed against your husband now; it's too late.
There's also another charge, more serious, that might apply. PC 288 is for intercourse with a child UNDER 14. So if the mother was under 13 at conception and 14 at the child's birth (not 100% clear in your question, so I figured I'd cover it). The statute of limitations for a 288 charge is up until the minor's 28th birthday. My math suggests that date has passed as well (15 + 14 > 28).
Discovery of new evidence can reopen investigations and in many civil cases can reset the statute of limitations, but in this case, the DNA test has no effect on whether the statutes of limitations for the offenses described above have passed.
Nicholas M. Loncar, Esq.
Your question is confusing to me at least. First if your husband was 19 and the young woman 14 when the child was born, the sex must have occurred while the young woman was under 14. If so this could give rise to charges far more serious than statutory rape. But then you add that the child is now 15. Huh? If so I would guess that any applicable statute of limitations has run.