The question of how long seadragons live has not been fully answered by scientists. Seadragons are hard to observe in the wild and therefore there are no direct measurements of their life span yet. In captivity, seadragons are known to have lived for up to nine years. This is considerably longer than their relatives, the seahorses, which usually live up to five years.
Seadragons are not seahorses, but they are closely related. Together with many pipefish species, they form a group of fishes that all have long tube-like snouts and have bony plates enclosing the body. The males always take care of the young.
It is fairly easy to tell seahorses and seadragons apart. All seahorses stand upright and have a grasping tail that they use to hold onto their surroundings. Seadragons, on the other hand, swim horizontally and cannot grasp with their tails. Also, seadragons have leaf-like flaps coming off their bodies that help make them look like seaweed. Finally, male seahorses carry their brood in a closed pouch, while seadragon males carry their babies attached to their tails without a pouch.
-- Josefin Stiller and Greg Rouse are marine biologists at Scripps Institution of Oceanography, UC San Diego
Above information is from link: https://scripps.ucsd.edu/news/voyager-how-long-can-seadragon-live-and-are-seadragons-seahorses