As chickens get older, the quality of the egg that they lay will deteriorate. Older birds will lay eggs that have more watery egg whites but the most common cause of watery whites is simply the age of the egg.
Storing eggs in high temperatures and low humidity will speed up the ageing process. Some diseases such as Infectious Bronchitis can also affect the ability of the bird to produce thick albumen (white).
You can test an egg for watery white, simply by cracking an egg onto a flat surface such as a plate and seeing how much it spreads out across the plate (or frying pan!)
There is a measure of how watery eggs are where the height of the white and the weight of the egg are measured to give a measurement in Haugh between 0 and 100. The lower the Haugh value is, the more watery the white is.
This measurement is only normally used in commercial egg production and the minimum value of 60 is usually required when eggs are sold in shops. Some older hens in a back garden set up will lay eggs well below this value.