There is no legal documentation of common-law in Canada, and there is no statute that says that common-law can only start at 18. In fact, in Canada, one can be married (legally) as early as sixteen. Essential to the understanding of "common-law" is that it is done without the benefit of authorization under a statute (law) - it's just a legal status that exists after a relationship has existed for more than 12 months. As the legal definition in Canada for marriage starts at 16, common-law can exist as early as that as well - but it's essential to note that there is no legal basis to define common-law. There immigration regulations that help a visa officer assess if a common-law relationship exists but they do not define common-law.
The difference between common-law marriage and statute-defined marriage is that marriages get certificates. If a marriage existed, and was legally registered, you'd have a certificate. If the marriage ended, you'd have a divorce certificate.
If a common-law relationship ends, you don't get a certificate. You just declare it ends and no longer cohabitate or live in a marriage-like relationship.
As to whether or not your wife "considered" it a marriage-like relationship, a party's consideration is likely not determinative. If the two people lived together, cared for each other, cooked each other meals, supported each other, did laundry for each other, etc., then it was a marriage-like relationship. Courts have upheld this - common-law applies even if you don't want it to apply.
You don't need to have any documentation as to whether or not a common-law relationship ended unless they doubt it did. Your marriage to your wife, along with her indicating the previous common-law relationship ended, is more or less determinative.