Those of you who have never had a truly excellent
peach probably have no idea what Eliot was alluding
to in this line. So, a little background:
A good peach is not eaten.
A good peach is something you wallow in.
You take a bite of the soft sticky sweetness, and you bathe in the abundant aromas and juices. As Prufrock is speaking of growing old at this point, he probably means that he can't trust himself to eat a peach withoug making a fool of himself. Note also how far Prufrock has fallen from his original lofty heights: He goes from "Do I dare disturb the Universe" to "Do I dare to eat a peach," all the while managing to feel sorry for himself. Prufrock at the end of his love song is beaten down by life, crippled by his indecisiveness and, ultimately, his fear of rejection.