This theme of the seven ages of man dates back to antiquity. In classical literature, though, descriptions of these seven ages were generally encomiastic, praising the virtues appropriate to each age and describing humanity as marvelous. Jacques, however, the speaker of this monologue in "As You Like It," is melancholic and misanthropic and thus emphasizes the negative aspects of each of the ages. Instead of describing infants as innocent and full of potential, he describes them as "mewling and puking" and schoolboys as whining. Lovers are absurd rather than romantic. 

The soldier, normally a figure one would admire, representing mature strength and devotion to duty, instead is described as rude and barbaric by Jacques, uttering "strange oaths, and bearded like the pard." Rather than considering soldiers brave, dedicated, and self-sacrificing, he portrays them as chasing after personal glory. The phrase "bubble reputation" suggests that rather than being motivated by a lasting sense of virtue or duty, soldiers are just seeking ephemeral fame. Risking one's life for a moment of fame (soon to be forgotten) is portrayed as rash and vainglorious rather than admirable by Jacques.  

Shakespeare adopts a playful tone to describe the "seven ages" of man. He does so to underscore the transient and ever-changing nature of a growing person.

Every stage of man’s life has its distinctive peculiarities. After passing through adolescence, a person reaches adulthood. He takes up a profession. Shakespeare, here, picks up a soldier as he best describes the temperament of a man in this stage of life.

When a person becomes a soldier or attains adulthood, he develops a heightened sensitivity to others’ opinions about himself. His concern for his reputation acquires far greater significance than anything else. He becomes so obsessed with his repute that he doesn't even hesitate to risk his life to win himself glory. Shakespeare puts forth this in an amusing way:

Seeking the bubble reputation
Even in the cannon’s mouth. 

"Bubble" signifies the idea of transience and impermanence. The “reputation,” for which a young man is eager to venture his life, is actually bubble-like; implying that it’s not going to last long.

It’s not considered prudent to endanger one's life for something that’s trivial or insubstantial. Life’s is too valuable to be lost for an insignificant cause. Shakespeare, however, isn't being didactic at all. He just points out, in a light-hearted manner, the peculiarity of a person at this stage of his life.