Idk what you consider full life span but just because it's body looks like it hasn't grown it's internal organs grow. This is called stunting a fish which can and does cause problems down the road. Why don't you do a google search on stunting a fish's growth and I'm sure you'll see that there's no healthy way to do that besides breeding programs.
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Any ichthyologist with a brain could tell you that fish are very adaptable and grow according to their conditions.