I purchased an 8" reidi as my first seahorse in 2000. He was an adult WC, and had been in captivity for at least 8 months before I received him. The day after I took him home, he gave birth to approx 700 babies. I "guessed" at his age at approx 1-1.5 years at the time.
I lost him earlier this year, after having him for 5.5 years. If my "guess" at his age was correct, he could have been almost 7 years old.
I recognized that he was getting "old" about 6 months before he actually died. Up until that time, he was producing babies about every 19 days. Prior to that, his batches kept getting smaller and smaller until eventually he stopped delivering at all. Like those fish described above, he appeared to have some vision issues, stopped hunting, and remained hitched to his favorite spot most of the time. I also noticed sunken places on his head before he died. I assume those were from lack of nutrition. Eventually, he just laid down on the sandbed and passed on.
His mate has about the same story, altho I purchased her about a year later. She was also WC and full grown, always very active. She agressively tried to "talk" her mate into moving around and eating, even going so far as to bully him into it, with snicks about his head. On his last night, she was never more than a few inches from him in a 75g tank. After he passed, she became very lethargic and adopted his attitude. She would not eat, and would remain hitched on his same hitch. I lost her about 2 weeks later. I'm guessing she was about 5.5-6 years old.