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."