how can i be a apple developer at 16

There are two aspects of the age limitation. One, as underage, you cannot legally sign a binding contract, and two, you need a credit card to sign up. But, that said, there is no explicit age limit to developer membership, just legal restricts that involve age related aspects.

So, what you need to do is get your parents to sign up, as they can legally create the account. And then you can use their account to develop to your hearts content.

Then the only limitations would be ones that Ireland places in terms of how old must you be to enter into a legal contract. As I said, there are two implicit age-related restrictions - one is can you get (or do you have) a credit card, and the second is are you old enough to independantly enter into a legal contract. If you are too young by either criteria, then you could still get your parents to sign up and let you use their account.