This sounds very strange to me. I would say, "till it was 15 years old". I think someone has made a mistake.
By the way, verbs don't have cases; they have tense and mood, among other things (Slavic verbs also have aspect). The standard term is "subjunctive mood". "Case" refers to nouns: nominative, genitive, etc.
There are some UK dialects, especially rural ones, which use were instead of was, and this was much more common in 1956 than it is today (though it's still common in some places). I suspect that's what we're seeing here. Is this the editorial voice, or is this some yokel speaking?