Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
This doesn't say anything about the gender or age of the beast, so can a druid change into a female bear if he has only seen male bears? Can he change into a bear cub if he has only seen adult bears? What about hens/roosters/chickens, which look quite a bit different? Or does the gender of the animal shape just match the gender of druid?
You just need to see one beast of its kind, not every possible phenotypic category of a kind of beast. If you did, it would specify that with something like
... the shape of a beast of a [gender/age/size/colour/seasonal pelt] that you have seen before.
It only specifies that you have seen that kind of beast before, nothing more.
I would make this entirely at the DMs preference. If a given druid had only ever seen adult dragonflies and wanted to assume the shape of a juvenile dragonfly, I would certainly allow it, but the outcome might not be exactly what the player wanted, because a juvenile dragonfly is not just a smaller version of an adult.
Likewise, if the character would likely assume a male black widow spider is just like a female except with boy parts, that's what he would look like.
I think it quite rare that fun opportunities like this would ever occur, and in most cases a baby animal looks like a cuter/smaller version of the adult and a female/male look pretty much the same minus the naughty bits, but I would certainly reserve this right for those rare occasions. At least make him describe how it differs from what he has seen in the past.
"A female red-winged blackbird? I dunno, a black bird with red spots on the wings. Just with a sexy voice and a milkshake that brings the boys."
