You can start evaluating at 3 days at least just for basic conformation but breed type takes longer! Usually, 8 weeks is the rule of thumb but I like to start at 6 weeks and go from there and yes, they do change a LOT! But the basics must be there for structure, conformation, quality of bone if its important in your breed etc., type is subjective and also the last to really develop. So what you choose now may not be all that you hoped for.
If you are using outcrossed bloodlines, expect more variation than linebred bloodlines though I do not advocate line or inbreeding for most breeders and if used by experienced breeders, used very judiciously with full knowledge of what is behind all ancestors of both pedigree sides!
I've evaluated many litters of different breeds for the past 45 years and still have trouble sometimes with my own breed because of attachments etc so that is when I asked trusted, knowledgable friends in the breed, over for additional set of eyes on evaluation.
Best of luck!