This same topic came up a few years ago, from that discussion below are my thoughts on the subject.
http://www.monsterfishkeepers.com/forums/showthread.php?388999-What-is-lifespan-of-flowerhorn
Unfortunately many FH are genetic weaklings, no different than other popular designer strains of fish, such as discus. There is hybrid vigor, and then there is just plain sloppy breeding where recessive genes start piling up causing genetically weak specimens.
Add to that people that insist on power feeding to the enth degree, and excessively high tank temps, to the point where this too can also lead to, or add to, premature death.
If you can get a genetically strong fish, one where the breeder didn't ignore health issues for certain physical traits, you should be able to get 8-10 years, or longer out of your fish.
Anything that creates stress on a fish will shorten its lifespan, including excessive breeding, or breeding before the fish is mature enough. Not just sexually mature, but physically mature in size & age. It's not a theory, it's a fact, especially with females that exert a great deal of their energy & resources producing eggs.
Also, modern flowerhorns can be just as long lived as some of the older strains, it depends entirely on the breeder. If overall health & vigor is of no concern, and the breeder only focuses on the physical attributes of his breeders, then genetically weak fish will generally be the end result.
This is exactly what happens with most of today's designer fish strains, not just flowerhorns. Very few breeders will spend a decade or more perfecting a line, or make health & vigor a priority, they simply pump out what looks & sells the best.