There is no straightforward definition of what constitutes a native speaker. This is partly because there's no straightforward definition of what constitutes a language. There are vast differences between the ability of even highly educated monolinguals to utilize the complete resources of a given language. Once you factor in education, register and dialect, you're in a right mess.
It is easy to differentiate between native and non-native speakers at the extreme ends of the scale but in between you have to look at:
- What is the purpose of using a particular language
- What domains is the language used for
- What is the social context of use
So for example, if the purpose of your determination was whether you can take part in one particular class of phonetic experiments, then just the fact that you're multilingual would be a problem.
But if you want to be a teacher or a translator, then things are much less clear. You can achieve native-like competence that would allow you to work as one or the other at any age. You can also lose that competence.
That's why nothing about your story can let us determine whether you should or should not label yourself as a native speaker. Have you continued using English regularly for a variety of communicative purposes? Are you confident in literacy-intensive environments? Would you classify yourself as being a speaker of a particular variety of English? Do you speak with a recognizably non-native English accent? Do you have intuitions about English syntax and morphology that converge with other similarly educated speakers?
You obviously have a reason to ask. If it's only a question about what you can put on your teaching resume, than I don't think you have to worry. There's no magic behind native speaker teachers. If it's a question that stems form a lack of confidence about your English facility then you have to make a judgement based on the situation. And say things like: "For the purposes of X, I am a native speaker of English. But if you ask me to do Y, I'd recommend someone else."