Embryos develop from the moment the egg is fertilized, which happens shortly after ovulation. This means that at the moment of lay, the egg is already 24 hours old and the embryo consists of 40.000 to 60.000 cells.
The optimum stage of development for storing the egg is the so-called gastrula stage, a specific stage of development which is very stable.
Young breeder flocks (in broiler breeders flocks younger than 32 weeks, in layer breeders younger than 27 weeks) produce eggs that contain embryos that are less developed. These eggs contain embryos that have less viable cells but are also more in the so-called pre-gastrula stage, the developmental stage prior to the gastrula stage.
In this pre-gastrula stage the embryo is less stable and cannot be stored that well. This means that eggs from younger breeder flocks are less suitable for longer storage than eggs from prime breeder flocks. When the breeder flocks are getting older (past 45 weeks) the embryo quality is getting down again and the eggs cannot be very well stored for a longer time as well.
The climatic conditions will have an influence on the stage of development.
Higher temperatures, for instance in litter nests vs roll-away nests, will allow the embryo to develop slightly more. This can have a negative effect in older flocks as the embryo might develop beyond the optimal stage for storage, but in very young flock it might work positive.
Also temporary heating of eggs before or during storage (Short Periods of Incubation Temperature during Egg Storage, SPIDES) can have a postiive effect on the possibility to store eggs. However, also here we have to be careful not to bring the eggs beyond the optimal stage of development.