I would imagine that the regular contract statute of limitations would apply here. According to
http://fair-debt-collection.com/statue-limitations.html, that would be 4 years in PA.
However, take a close look at the fine print at the end of your lease. If it says that the lease is a "sealed instrument", or words to that effect, the limitations period would be 20 years instead. (The concept of "sealing" a contract is pretty archaic, actually, but still has the occasional side effect even nowadays.)
I found the following web page setting out the Pennsylvania law relative to security deposits:
http://www.mv.com/ipusers/nhpoa/padepo.htm. This law doesn't appear to have any special provisions for a tenant to contest the statement of damages. A local legal aid office, or even the clerk at your local courthouse, might be able to give you more information.