Given that she had at least 2 keepers, her actual age could be just about anything. It sounds to me like she was just old and probably very old at that.
Generally it takes about 5+ years for a B. smithi to even reach adult size. After that, they can live for upwards of 20 years.
Some live longer, some not as long, but the "average" life span of a female smithi is about 20 years.