I understand many under 16's would abuse this and by the animals for a cheap laugh, but then again, people like me are perfectly capable of raising and caring for a fish, or indeed any other animal in my opinion.
That argument just doesn't work, mainly because of other laws regarding age. I'm sure there are some 14 year olds that can handle driving, or 16 year olds that would be responsible drinkers, but it's done to protect the majority of people. The law is because they think not all, but the majority of people under 16 aren't able to fully recognize the responsibility they're undertaking by purchasing a live animal.
I agree with the law. I think that if you're really serious about it and under 16, your parents will go with you and help you out.