According to our current understanding of physics, photons, which are particles of light, are believed to always travel at the speed of light in a vacuum from the moment they come into existence.
In the framework of special relativity, the speed of light in a vacuum, denoted by "c," is considered to be the maximum attainable speed in the universe. Massless particles, such as photons, are inherently constrained to travel at this maximum speed. This means that photons do not need to accelerate to reach the speed of light; they are created already moving at that speed.
When a photon is emitted, such as from an atom transitioning from a higher energy state to a lower energy state, it is considered to be "born" at the speed of light. It does not experience acceleration in the conventional sense because it is massless and moves at its maximum velocity from the moment of its creation.
It's worth noting that photons can experience changes in their direction or wavelength due to various phenomena like refraction or gravitational lensing, but their speed remains constant at "c" in a vacuum throughout their existence.