If in 1900 the minnesota supreme court ruled that a minor can void a contract at any time during minority and, in 2014, the courts in minnesota still follow this ruling, this is an example of

This would be an example of: Stare decisis
Stare decisis is a legal doctrine which make a certain court would generally follow the application of the similar case that already tried in the past.
For most cases, only higher courts usually break the status of stare decisive and it's only if there is some sort of progress/advancement in the law regarding the case.