Why do Identical twins differ from each other over the years?

Genes – which are the basic building blocks of living creatures – contain the code for our life.  But genes are the "hardware".  Epigenome are the software.  As Vish explains here beautifully:

In the genetics world, the genes are the “hardware” while the epigenome – a set of chemical tags which can turn the genes on/off or change their collective behavior – is the “software”.

This is the reason behind the differences in identical twins.  And how are these epigenomes handled?

Because, like the wise men on the ancient world- Socrates and Charaka (of Ayurvedic fame) used to say: “you are what you eat”.  The food we eat has a profound affect on the epigenome, not only our epigenome (our software) but that of future generations.

This link on PBS gives more information on epigenetics – which is a field different from genetics.  Here are some interesting things:

chemicals (that) may have an epigenetic effect, such as methyl groups, can enter the body from one’s environment. Consequently, an individual’s choices and personal history, such as diet and smoking, can influence his or her exposure to epigenetic triggers and their accumulation in the body.

and

For example, one set of genetically identical mice was fed food rich in methyl groups. The methyl groups bonded to the mice’s chromatin and blocked the expression of certain genes. This epigenetic effect produced two sets of mice that looked extremely different.

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