Its largelly about the money. Although generally with more agressive types of fish raising together from young can cut down on problems. Theres the simple economics of transport and holding and feeding costs, its far cheaper to breed fish and raise them for 2 months before shippping them out 200 to a box than to raise a fish for 2 years then ship it out 20 in a box, quite often juvenile fish will deal with the global transport better than adults as well, bear in mind that most of the fish you will buy will come from asian farms and have to spend 24 hours+ in a box with no filter heating or air supply! You wouldnt want to think of the cost per fish if everyone were having to raise and ship adult fish. Besides if youbuy young you know that you have the full life expectancy of the fish aheadof you instead of not knowing if you are buying ex breeding stock in its retirement years.