Do you understand the difference between fact and fantasy?
I'll take that as a "no" answer to my question, then. No worries. I'll go over it in brief.
Sex is a bimodal, biological form of classification, primarily defined by a collection of biological traits. In humans, it tends to be divided along male and female ranges, with rare instances of intersex. Males are generally defined by such traits as XY chromosomes, external genitalia, and sperm and testosterone production. Females by such traits as XX chromosomes, internal genitalia, ovum and estrogen production.
Gender, meanwhile, is a series of social labels that, while derived from biodimorphic associations with sex, are distinct from sex in terms of category. It encompasses a wide range of social functions, including pronouns that we use to refer to each other and a vast array of social expectations or restrictions (often depending on social context).
For example, when a doctor looks at a newborn baby's genitals and says "it's a girl!", they are identifying sex. However, following this identification, it is determined that, because this baby is a girl, it must wear dresses, play with dolls and grow up to be a housewife. THOSE things are the domain of gender; they are social roles and expectations that we tend to ASSOCIATE with a given biological sex, because for countless decades being born a particular biological sex made it almost mandatory that a certain set of these such expectations would be expected of (if not entirely forced on) you. Because these associations exist, we cannot avoid forming certain expectations and associations of people purely by what gender classification they fall into.
Now I have hopefully brought you up to speed, I will answer your question:
Yes.
If one can change their sex by wishing it to be so,
Not sex. Gender. They are two distinct things.
then one can wish to identify as a different age, different species, etc., and everyone must play along with their fantasy and ignore the facts, right?
No. Because gender is a social concept, whereas age and species are not.
If you want to join in this discussion, at least make an attempt to start with a basic grasp of what it's about.