Yes, the portrait is from 1878. However, it is taken from images from the wedding, which took place March 10, 1863. Alix's gown was made by the House of Worth. She was 18 at the time of her marriage.
The Marriage of the Prince and Princess of Wales (later King Edward VII and Queen Alexandra)
William Powell Frith
1878
Painting - oil on canvas
Height: 60 cm (23.62 in.), Width: 48 cm (18.9 in.)
The Royal London Hospital Museum
image via the BBC