I can't stress this enough....the RCMP, Police, CSIS, and other law enforcement agencies can see this on your record for life. It's just when you turn 18, the information is essentially sealed from the public, etc. so it doesn't affect you when applying for jobs, etc. Even the border patrol can see your history as a friend of mine who had a run in with the law at 16 was questioned about it when entering the states at the age of 22. He has had no other run ins with the law.
My advice to OP is to call and explain to them the situation. Sure there is a chance it will go through if you don't say anything but you stand the risk of getting denied for not being truthful on your application. It will be a difficult argument for you to say that you forgot how long it had been since your offense as most people would likely make sure of the dates before applying for a PAL.
Either way, good luck OP!