The crystalline lens differs from other biological systems. Normally, epithelial tissue ejects older cells, but as lens fibres are formed, the older fibres are pressed into the nucleus. The continuously increasing lens proteins, soluble at the periphery, change to insoluble proteins in the nucleus which hardens as compression intensifies. With age this process changes the lens, making it more resistant to molding, until it becomes only about 25% of that in childhood.
A contributing factor is exposure to near ultraviolet of 310-400 nm. It initiates a process of photo-oxidation which causes a discoloration of the nucleus, from yellow to brown, with increasing loss of transparency.
However, another important factor is environmental temperature. In coastal regions where there is less ultraviolet but higher temperatures than in mountainous regions at similar latitudes presbyopia starts earlier. The external temperature is transmitted to the lens through the cornea.
By way of examples one study has shown that the mean age of presbyopia in Norway was 46 years, in mountainous Ecuador 42.5 and coastal Ecuador 39.7.