Just agreeing with and adding to China's estimate, the lady's hair is covering her ears, and this was a style popular in the early 1860s - around 1865 hair started to be pulled up a bit higher, gradually revealing more of the ears until by 1870 the whole ear was on show.
The style of portrait and the clothing worn all point to early 1860s. No earlier than 1860 because these photographs were not around before that!
Cheers
Prue