One of the trickiest aspects of hawk identification is telling adult female from juvenile Northern Harriers at a distance. Most people are familiar with the rusty colored, essentially unmarked underbody of juveniles versus the paler buffy, streaked underbody of adult females. However, adult females can show rusty undersides (more commonly in the West) and appear very similar to juveniles, and the underside of juveniles fades to buffy by winter/spring and can appear similar to that of an adult female.
Before "clicking" on the composite above to enlarge it, note the overall color of the 4 birds as they would appear in the field. The 2 left-hand birds appear orangey underneath as you would see on typical juveniles, and the 2 right-hand birds appear buffy resembling adult females. Now enlarge the photo and see that the 2 adult females (1st and 3rd bird from Utah, November) show streaking on the body, and the 2 juveniles are basically unmarked on the body. The 2nd bird was photographed in September and the faded juvenile (last bird) was photographed in December in Utah. Consider that, if the lightly marked adult female was an orangey type, it would be nearly impossible to tell from a juvenile without ideal views.
Yes, there are other plumage differences between adult female and juvenile Northern Harriers (topside color, marked undertail coverts, head pattern, etc.), but they are minor and difficult to pick out without considerable experience. Telling Harriers from other hawks can be difficult, so classifying a "brown" Harrier as unknown age/sex shouldn't be a bother. Too often, I have seen birders 'shoot from the hip' when ageing Harriers never aware they were incorrect.