In the name of Allah, the most compassionate, the most merciful
Although some scholars believe that it is mandatory to slaughter (sacrifice or aqeeqah) at the 7th day of the birthday, but in accordance with the view of most scholars , it Is Mustahab to slaughter(doing Aqeeqah) a sheep at the 7th day of baby’s birthday. Of course seemingly they emphasized the significance of it anyway.
According to some traditions, the best animal for the Aqeeqah is “Sheep”, then camel , then every animal which is possible to be sacrificed..
It is Sunnah that you do it even till end of your life ( in the case that you couldn’t do it at the 7th day), hence if you couldn’t give any Aqeeqah till 7th day, then do it whenever possible.
About the gender of the sheep (for the Aqeeqah), Imam Sadegh (a.s.) mentioned that if the newborn was boy, the sheep should be male, and if the newborn was girl, the sheep should be female.(Of course seemingly, it is not mandatory, but seems to be better if the gender…)
 Al-Entesar Fi Enferadat Al-Emamiah p:406 الانتصار في انفرادات
الإمامية، ص 406
 Al-Helli, Al-Allamah, Hasan ibn Yusf ibn Motahar Al-asadi …
الحلّي، العلّامة، حسن بن يوسف بن مطهر الاسدي،...
(To see the complete text of the mentioned reference, you can see the source.)