Dear Villager -
Welcome! Here is my page on Madeira -http://www.wineintro.com/types/madeira.html
Madeira isn't really a wine. It's a mixed drink. It's wine-plus-brandy. The way they make it is they have big vats and they use previous years' mixture to then add to the current year. So there isn't ever a vintage. It's a mix of all sorts of things. Usually what they'll state on a madeira bottle is its "minimum age" - that it has at least 5 years of aging on it. But because even that first year probably had some older previous-year mixture in it, it's usually older than that.
So they don't put "a year" on a bottle and that would just be the year the mixture happened to be bottled, not the age of the stuff IN the bottle.
Your only hope might be that they occasionally changed the label and to write the winery and asked what years the label you have were used during. If you really care
I would soy to drink it with friends and enjoy! Either it's good or it's way past its prime. Either way it'll be an adventure - let us know what you think!