If we're looking worldwide, then the question can be narrow down to "Where has there been a continuous legal system the longest?" The UK ( or more specifically, England and Wales), are a good bet for this, due to general lack of upheaval, revolution, invasion, etc in the last 1000 years. With some quick research, the oldest law in the UK that has legal relevance are two sections of the Statute of Marlborough, passed in 1267. Of its 29 chapters, only two remain with legal force: a recent review of statue law repealed some other sections of it, but explicitly kept some with legal force (Page 108). These are known as the Distress Act 1267 and Waste Act 1267 respectively, and their full text can be found here and here. Essentually, they outlaw private feuds or taking stuff from your neighbours without the courts permission, and that if you are a tenant, you can't trash or sell the property you are a tenant of, and will have to pay full damages if you do.

There may be some aspects of islamic law which are still obeyed today but were ruled on further back, but I would not know, and then you are blurring the line between religion and government.

If Canon Law is included -- and there are good arguments as to why it should be, given that it remains relevant both for Catholic purposes worldwide and within the state of Vatican City -- the twenty canons adopted in the First Ecumenical Council of Nicaea in 325 CE would be the answer to this question.

With some amendments, the twenty Nicene canons are still in force.

Interestingly, Canon 1 prohibits self-castration for Christians.