I went into some research of cows and it seems that it depends largely on the cow breed. There are cows that are not rarely able to productively give milk up to 14 years of age (first calves come at ~25 month). What I found is that you can't expect your cow to be milkable that long (especially considering the regiment those cows go through in factory style milk production). Usually already at a considerably young age them being able to produce milk is irrelevant as long as it's not enough anymore to validate not slaughtering the cow. What I wanted to point out though, it is definitely not impossible, to have a milkgiving cow of 12 service years.