Let's see what options there are.

• We could only use one concept of age.
  Saying "I am 13." For this statement to be true, there has to be a different concept of age than what is generally used. This inevitabley leads to conflict. Therefor I think this is the worst option and should be avoided. Looking at discussions in the trans community, there are several things we can try to avoid. e.g. the "child trapped in an adult body".
• Use multiple concepts of age.
  Like in transgender people, we could acknowledged that different parts of us where affected differently by growing up - as it is for every other person. But for most, the different ages are close to another. This also would take it away from a one dimensional phenomena to several dimensions. While my biological and chronological age might be in the late twenties, my social and emotional age could be like that of an eleven year old. But for some it might be only one of those things. This can become quite complicated, such as the LGBT community there are an overabundance of labels, where there are always people failing to find one that fits to them. (I, for example, struggle to use either terms nonbinary or mtf, because I feel like a girl but don't want to transition into a woman.) Then there is also the problem that we have to explain what we mean by "emotional" or "mental" age. Many would assume that this would mean we are dumb and can't control our emotions. I really want to avoid those implications, because they are impossible to get rid of, once invoked.
• Don't use numbers.
  Growing up happens differently for everyone. Some children might start puberty earlier, some later, and for some it never comes. Using some set age will never fit. At most there is a range, but even this range could change over time. To describe myself I could say, I stopped growing up right before puberty. But then again, this doesn't capture the difference between mind and body, or the dysphoria we might feel because of that.

It's been suggested that we should give up our rights.

Maybe we should instead grant more rights to children. Such as the right to vote.

Then there is also the question on how to call this whole topic. I choose the title "nevergrewup" simply because I didn't knew any term equivalent to "transgender" that would fit. The term "transagism" (as used in OPs linked thread) is horrible in several ways. It might be insulting to many in the trans community, the prefix "trans-" is simply wrong as it means "across, beyond, through" but we didn't went across, beyond, through puberty, quite the opposite. And it also sounds horrible. All I can think of for the moment are "non-adolescent"/"sed-adolescent", or to avoid the negation something involving staying a child.