Jaques' essential point in his "seven ages of man" speech found in Act 2, Scene 7 is essentially to assert that life is rather arbitrary. He compares life to a play with all of humanity participating as its actors. Plays too are rather arbitrary. They are important within the moment they are read or performed, but beyond that moment, they serve no greater purpose. Hence, by likening life to a play, Jaques is also saying that life has no greater meaning beyond the point that life is performed, showing us his assertion that life is arbitrary. Hence, the central theme found in his speech is the arbitrariness of life. But as he continues to speak, he also relays seven different common stages of life, performed like seven different acts.
The first stage he starts with is of course infancy, then he describes a "whining school-boy," and a "young lover, / Sighing like furnace," which most likely describes a boy's teenage years or very young adulthood. The fourth stage he describes is that of a soldier, and that's because most, if not all, young men are expected to enlist in the military. Hence the reference to a soldier describes the stage of life in which a many is a fresh, young man. Next, he likens middle age to a "justice," or judge, because in your middle ages you are much more full of wisdom and able to judge than you were when you were young. Sixthly, he moves into a description of an elderly man "[w]ith spectacles on nose and pouch on side." Finally, the seventh stage he describes is the stage of death, which he likens to a second childhood, or infancy, because once again at death a man is without teeth, eyes, taste, or anything. We especially see him likening death to the final lines in his speech:
Last scene of all,
That ends this strange eventful history,
Is second childishness and mere oblivion,
Sans teeth, sans eyes, sans taste, sans everything. (II.vii.163-66)
Hence, essentially, Jaques describes the seven stages of life as moving from infancy and back again to a final infancy in which you are once again left with nothing in order to further portray his theme concerning the arbitrariness of life.
Nothing is permanent. Everything in life will eventually move on. Death is the exit and birth the entrance.