You could either key out the species and find out that way (not necessarily an easy task, given that there are over 2,000 known species of Ophiuroids), or you could attempt to estimate the specimen's age (this is typically done by looking at the growth rings on the vertebral ossicles of the brittle star, which, unfortunately, typically requires both cutting out a section of the star's arm and the use of a Scanning Electron Microscope; this provides estimates, not exact ages, as the growth rings are thought to be seasonal, possibly somewhat dependent on nutrition, and to have age-related differences/changes in growth).
If anyone wants to try keying out the species, the following link should be able to help you narrow it down to the family level:
https://www.researchgate.net/public...a_leads_to_revised_morphological_descriptions