The anal tuft of hair hangs down below the anus of female Roe and lengthens during Winter, as does the male equivalent tuft on the penis sheath.
It is a very valuable identifier after the Bucks have cast but before the new antlers become noticeable.
The male’s tuft can be hard to see unless the animal stands at precisely the right angle but the female’s is easily seen as long as her rear end is visible.
The later in the season, the longer the tush.
As to the age at which Does develop it, I think we are fairly safe to say that it will probably be in their second winter when they are about 18 months old.
They were kids in the previous winter and although animals, like humans, can be precocious, they are usually easily identified without reference to antlers, tushes or penis sheaths.
From the deer management point of view, the problem with the anal tush is that by the time you have got a good look at it the Doe will be facing away.
Then one must wait for her to turn broadside again – or risk an “English Brain Shot” which is effective but can damage a lot of meat.