What is considered the first stage of the cell cycle?

The first stage is the Interface.

Explanation:

There are primarily two main stages in a cell cycle; but the first stage is the interface. This is a stage in which a cell grows and also replicates its DNA.

It can be said to be the longest phase of the cell cycle. During this phase the cell growth reaches its maximum size, replicates its DNA, prepares for cell division, and also performs it very cellular functions.

This stage has three parts which include: G1, G2 and S phases.

Some cells do not need to divide to exit the cell cycle. These cells can exit the cell cycle permanently. Example is a neurons, they may also exit the cell cycle temporarily. These cells are said to be in G0 which is not a stage of the cell cycle.