I am going to take a different tack than many here. I bought my first fish at about 8 years old and it got treatment as good as the hobby knew how to give at the time, not up to today's standards. I have known people well over 30 who try to get away with uncycled tanks and just feed a bit of flake every day. They know nothing, or care little, about water changes and similar subjects. Although an age limit will mean three is a better chance of the buyer being somewhat mature, maturity is not a function of simple aging, it is a function of learning and the environment a person grows up in. Even with an intensive quiz into fish keeping principles before a sale, there would be no guarantee the person would go home to apply the standards.