Grab this URL : http://csmg.lgmobile.com:9002/svc/popup/model_check.jsp?model=LGD800&esn=ADD_IMEI_HERE

Now, replace the LGD800 part with your model (LGD802 if you have the D802, for example) and ADD_IMEI_HERE with your IMEI

Open the URL

A list should appear

esndate = the date when manufacturing of the phone begun (probably)
esnoutgodate = the date when the phone was done being manufactured (probably)