If we talk about the age of the universe , I am sure that you would know better how the universe created and how its age is defined. Now if we talk about the expansion than you would be able to know how it is created When it happened and What happened even what is happening now? Yes, the universe is expanding now also, I know you are surprised a bit and very few are familiar with this fact that the universe is expanding every time. though it is happening is very slowly, Everything in the universe is moving farther away from everything else.
In the process of expanding the time period of expansion is divided into three periods
The Singularity Epoch
The singularity epoch is the earliest known period of the universe. At this time, All matter was condensed on a single point of infinite density and extreme point of heat. During this period it is believed that the quantum effects of gravity dominated physical interactions and that no other physical forces were of equal strength to gravitation.
The Inflation Epoch
With the creation of the first fundamental forces of the universe, The inflation epoch began fasting from 10^-32 in planks time to unknown. Where the phase transition that caused the separation of forces also led to a period where the universe grew exponentially. It was also at this point in time that the imbalance of matter and antimatter occurred, Where the temperature was so high that random motions of particles occurred at relativistic speeds.
As a result of this, Particle and anti-particle pairs of all kinds were being continuously created and destroyed in collisions.
The Cooling Epoch
After the inflation stopped, The universe began to cool and matter coalesced and formed. As the universe continued to decrease in density and temperature, The cooling epoch began. Since in inflation because of very high temperature, The universe becomes unstable just after 10^-11 sec. After the big bang, particle energies dropped considerably about 10^-6 sec, The quarks combined to form baryons such as protons and neutrons. Since the temperature was not high enough to create new proton-antiproton pairs, Mass annihilation followed immediately, just leaving all original protons and neutrons and none their antiparticle.
This is how the expansion happens in the universe, neutrons, and protons combined to form the universe’s first deuterium and helium atoms.