From what I know, few PhD students at top programs (Stanford, Wharton, MIT..) have any professional experience in accounting. It becomes more common as you go 'down' the rankings, e.g. as you get into good state schools with a more important 'professional school' component (CPA preparation), such as Illinois, UT-Austin, and so on. They're not mutually exclusive but I know for a fact that some elite research schools could really care less about Big 4 experience, while those who want to graduate good teachers as well as researchers will give more weight to this type of professional experience.
To get back to your initial question, I don't think it can hurt (from a PhD admissions perspective) to have some work experience, but if you have 10 years of w.e. in an unrelated field, you need to address that in your SOP. On the other hand, to have work experience may also stop you from saying extraordinarily stupid things in your SOP as well (like "I think all auditors are corrupt and my research as a PhD student will show this").
The reason why you see that most current PhD students are 25+ is not always because they worked for a few years before going back to school to do a PhD (though it's my case), but also because most of them have Master's degrees (Econ, Statistics and so on) that took them a few years to complete before they got into a PhD program. A grad degree in a relevant field (with good results) is much more important than work experience (say Big 4) when it comes to evaluating research potential.