Your question does not specify whether the arrears owing to you relate to: 1) spousal support; 2) child support; or 3) both. Also – importantly – you do not indicate whether these arrears have accumulated under a Court order, or whether you have just done your own informal calculations as to how much support you think you should have been paid over the years.
If there is currently no Court order in place, and you are now trying to get an order that is retroactive in nature, then you may have trouble obtaining some of those arrears due to the long delay in trying to collect. You should speak to a lawyer about the specific details of your case.
However, if there was indeed a Court order issued about 18 years ago, but has been ignored since, then you are in a much better position, even after this long period of time. Under Ontario law, there is no deadline (or “limitation period”) for enforcing a Court order. So you are free to take enforcement steps, which for Family Law matters might be achieved through the assistance of a lawyer, or through the Family Responsibility Office.