Jaques' "Seven Ages of Man" speech is fantastic, and one of my favorite Shakespearean speeches. There's a great example of alliteration toward the end of the speech, but, before we can look at the example, it's important to remember what alliteration is. Alliteration is the repetition of a particular sound at the beginnings of several words in a particular grouping, such as a phrase or sentence. It's worth pointing out that alliteration is based on the repetition of sound and not necessarily just letters; the phrase "fun philosophy," for instance, uses alliteration, as both "fun" and "philosophy" utilize an "f" sound at their beginnings.
With that definition in mind, take a look at the following lines toward the end of the "Seven Ages of Man" speech: "His youthful hose, well saved, a world too wide, / For his shrunk shank" (22-3). This section of the speech employs two kinds of alliteration. First, there's the repeated "w" sound in "a world too wide"; then, there's the repeated "sh" sound in "shrunk shank." Alliteration is easy to spot once you get the hang of it, so I'd encourage you to read the rest of the speech to see if you can find any more examples of alliteration.