Well, it depends. The common recommendation is around 6 months. This is because they have good head and torso control at that age, so it becomes very easy to put them on your back.
On the other hand, lots of cultures wear newborns on their backs. It is very possible to do, but requires some knowledge and techniques to do safely. Because we're not used to that in western europe/north america, people think it isn't safe.
So. If your daughter has good torso and head control, absolutely you can wear her on your back at this stage. If she doesn't, but you are brave and careful (

) you can wear her on your back anyway. At this age I bet she'd love a high back carry. Tie your babyhawk waist straps above your breasts, have someone put her against your back, pull it up and over her back, bring the straps over your shoulders, under your arms, and tie them behind her back.
If you don't have someone helping, this is much trickier, of course.