If a creature is a Wild Card, are its young also Wild Cards, or do they only become Wild Cards once they have matured?
Whether something is a Wild Card or an Extra is a matter of game plot and story, not about the fictional game world. It is entirely possible to have adults Rocs that are Wild Cards right beside other adult Rocs that are Extras, so long as that makes sense for the encounter design. The default Roc in the book happens to be a Wild Card, but it's not a rule that they must be. (Ditto, not all goblins must be Extras.)
A creature or NPC is a Wild Card if you, as DM, are intending them to be a major challenge. The rule of thumb for whether you should make something a Wild Card or not is, if you want the dice to decide whether they survive or are defeated by the players, then you can make them a Wild Card. If you intend for the creature to go down, they're an Extra even if the stats in the book say they're a Wild Card.
Essentially, is this an important character in its own right, or a speedbump? Important characters will often (but not always) be Wild Cards, while speedbumps are always Extras.
So, are these young Rocs supposed to put up a nasty fight and have a good chance of surviving encountering the PC group? Then they're Wild Cards. If the young Rocs are just meant to be cannon fodder (even if powerful cannon fodder) that will make the fight with the adult Roc more challenging, then they're Extras.
This isn't explicitly covered in the rules, but my interpretation as a GM is as follows.
A Wild Card is tougher to put down, having three wounds rather than the one that an Extra has. They also are much more capable with their Attributes and Skills, as they have a Wild Die with all their rolls. Remember that making something a Wild Card increases their chance of success from 50 to 75% with a d6 and Target Number of 4, which is a significant increase.
Based on the above differences, I think it would depend on the creature you are talking about and whether you are giving offspring lesser stats than their parents as they grow up.
Ask yourself whether it makes sense for a baby creature to have those three wounds. Is the adult creature a Wild Card because it is generally intended to be a super nasty, and is the baby already nasty, or is it cute and fluffy until it grows up. If it's nasty from birth, I would make the offspring a Wild Card but also reduce their stats until they are fully grown. If not, then I would start them as an Extra, and make them a Wild Card on reaching maturity.