In Shakespeare's As You Like It, what is the effect of the repetition in the last stage of man as described in Jaques' speech?

The repetition at the end of Jacques' "Seven Ages of Man" speech has a pessimistic, even nihilistic, effect. Jacques is essentially arguing that people re-enter childhood when they are old, only this time, they are far more pathetic, having once been active. In Jacques' view, old age is associated with decline and indignity, and therefore, life itself lacks dignity, since we all end up where we began: helpless and ignorant, then completely unaware of existence and the senses. However, it must be recalled that Jacques is pessimistic and his view is not necessarily shared by the other characters in the play.
Interestingly enough, when Jacques completes his speech, Orlando enters with Adam, his aged and faithful servant, on his back. Adam is treated with respect by Duke Senior and the others present, called "venerable." And considering his devotion to Orlando and his helpful ways, he is hardly infantalized the way Jacques characterizes the elderly, even if he is not as physically strong as he once was.

In Act 2, Scene 7 of As You Like It, the melancholy Jaques muses on the stages of life. The speech begins with infancy, with man "Mewling and puking in the nurse's arms" (144), and then it ends in a similar fashion, as Jaques claims that all of us return to an infantile state:

... Last scene of all,
That ends this strange eventful history, 
Is second childishness and mere oblivion;
Sans teeth, sans eyes, sans taste, sans everything. (163-66)

Jaques' speech essentially muses on the ridiculousness of human life, as each of his seven stages can be seen as pathetic and bereft of dignity. The repetition in the final stage, in which man becomes a pathetic copy of his infant self, is particularly pessimistic, as it argues that the average human life accomplishes very little and all of our efforts simply take us back to the state in which we began. As such, by repeating the first stage of man in the final stage, Jaques underscores his pessimistic worldview and suggests that the course of human life is pathetic and full of folly. Of course, this opinion is Jaques' personal worldview, and so it isn't necessary to take it completely to heart. Indeed, the play's happy resolution does something to destabilize Jaques' pessimism. Be that as it may, Jaques' "Seven Ages of Man" remains an important and thought-provoking speech.  
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