I think you so must go through the poem once again throughly (line-by-line. Search for the meanings if u don't understand and take a good grasp of it.

here is the theme given in the solution section of this website:

One of the main themes of "All the World's a Stage" is the fleeting and unpredictable nature of life. The metaphor of the world being a stage and people being players suggests that life is like a play, with people taking on different roles and facing different challenges and experiences as they go through different stages of life. The poem describes seven different stages of life, beginning with infancy and ending with second childhood, and suggests that people's roles and experiences in life are temporary and that they are not in control of the events that happen to them. This theme is reinforced by the use of similes and metaphors to describe each stage of life, which help to convey the idea that life is a fleeting and unpredictable journey filled with a variety of different roles and experiences.

Another theme of the poem is the idea of detachment and detachment from the events of life. The metaphor suggests that people are simply going through the motions of their various roles and experiences without any real control over what happens to them. This theme is reinforced by the use of language that suggests that people's roles and experiences in life are temporary and that they are merely players on the world stage.

Overall, the poem uses the metaphor of the world being a stage and people being players to explore themes of the fleeting and unpredictable nature of life and the idea of detachment from the events of life.