i see that sprite has a band - so that should have the birth date, doesn't it?
i won't venture to guess an age of neige especially since it is albino - ..
one suggestion - i would hold off on converting to pellets - you have the rest of their lives to switch them to pellets. for now, if you are concerned that neige is having some trouble feeding for herself, i would get her on whatever seed she was eating before you got her - make sure that she appears to be hulling her shells and eating ok on her own. i have no experience with pellets, but i have heard that birds can have difficulty with them. crushing the pellets may be helpful, but as i said, for now i wouldn't bother until you know both keets are thriving.