Born With H.I.V., Struggling With Teen-Age Lives
Report on plight of children born with HIV who are now, because of medical advances, living far longer than anyone expected, and thriving into their teens and beyond; as these children grow, normally turbulent issues of youth--like dating and independence--are colliding with their chronic terminal illness in ways that their families never envisioned when children were babies; until early 1990's, HIV-infected children were considered lucky to make it to first grade; but their life expectancy has been so extended that they are graduating from high school; AIDS deaths dropped by 50 percent in children from 1996 to 1997, reduction that Federal health officials say is probably result of combination drug therapies and medicines to treat opportunistic infections; examples of some HIV-infected teenagers, and their families, discussed; photos (L)