I have something simple to say.
Age is not a boundry.
Maturity, research, and such Is what matters.
You should have to take a test on a fish you have to buy, and also have proof that you have adequet housing.
That is my opinion. a 35 year old man is just as likely to abuse an oscar as a 10 year old kid.
A 25 year old mom is just as likely to put 3 goldfish in a 10 gallon as a 15 year old teen.
True, but Situation and Circumstances do matter- you may do all the research in the world and treat your fish as best you can, but there's no point in taking up the hobby if you have to abandon it somewhere soon down the like due to change in situation or circumstances i.e. like having to go to Uni/college/boarding school, moving out of home, lack of money to pay for hobby etc.
Before 16 years of age, you are unlikely to have a job or any serious say in such matters as what course of life you take- you have breakfast, go to school, come back and eat, do as your parents say, tidy up, go to bed, repeat similar process next day sort of thing etc.
But at age 16 you are old enough to start taking on adult responsabilities- you can start learning to drive, move out of home, get a job, legally have sex etc. You may not have the maturity of an adult, but you will have a lot more control and responsabilities over what happens in your life- and which such changes, comes the responsability of true ownership over an animal.
I think its generally advisable not to take on the responsabilities of an animal if you don't know what you want to do in life or don't have very stable circumstances/situation to bring a pet up in.
For example, if i had got my first hamster before i moved out of home, i would not have been able to rent out the place i did when i did- the hamster would have had to stay at home (which would have been bad as i know my mother would be incapable of looking after an animal like a hamster since she hates rodents) as it was against the landlords rules to bring in any furry animals.
If i had bought a 55gal tank before i moved when i upgraded to a larger flat to rent, i would not have had the time to look after such a large tank properly etc.
So yeah...Basically, you can be a great fish keeper, but if you don't plan ahead enough and consider wisely you current and future situation, you could end up in a situation where you can't even be a good fish keeper anymore because you can no longer continue the hobby due to various circumstances etc.
Sometimes its better to wait for good things than to rush into them at a young age

. I myself am certainly glad i took up my hobby when i was 16 rather than before as i know how much more difficult it would have been with my mother (and for others its vice versa).