I`ve always assumed Sarabi is older than Sarafina, if just slightly. Sarabis darker fur might not be related to her age at all, but she just seems to be such a mature and wise character, and wisdom comes with age, right?
In any case, my personal opinion is that Sarabi is slightly older than Sarafina, but we might not be talking more than weeks.. I don`t think there are several years between them. I`ve always imagined them being friends since cubhood, so they would have to have grown up together, and therefore born only a short time apart.