As we covered in Emily's first fic, Andy's third fic (Barbara saves the day on Birthday), as well as elsewhere, telepaths in the Corps celebrate a collective Birthday holiday on April 12th, the day the Corps was founded. (Birthday is really big deal for children, but celebrated less among adults in the Corps. YMMV.)
For reasons never sufficiently explained in canon, telepaths count age by a variant of the traditional East Asian age reckoning system - although babies are born at "0," they all collectively age a year on April 12th, and it is this age reckoning system that is used in the cadres and in telepath schools. The reason for this practice may have both to do with Asian influence on telepath culture (the Corps extends across all EA jurisdiction, after all) as well as with a desire on the part of the Western founders of the Corps to build a "collective society" among telepaths, one that places less emphasis on the individual and more emphasis on the community and "the family" (the Corps). Some of the telepaths who became influential in the early days of the Corps had themselves already embraced a "collective ideal" for telepath culture, based on the view of all telepaths as "kin and kith" (a view maintained in the days of the Corps, and expanded upon under the political metaphor of the Corps as Mother and Father, which is also Confucian in its influences). As all telepaths are "children of the Corps," it is the Corps' birthday that "matters" over the birthdays of individuals. (The holiday is called "Birthday" as if the Corps is a "person," and children celebrate it as their own personal birthdays, but it's also a celebration of the establishment of the quasi-nationhood that is the Corps. The personal and the political are inseparable.)
Generally speaking, the East Asian influence on telepath culture is occasionally mentioned in canon (the repeated mentions of "Rashomon" come to mind), but telepath values reflect East Asian influence more often and more deeply than canon directly specifies. This post discusses just one of these aspects - I explore a second aspect here (an excerpt from the Psi Corps Student Handbook about sacrifice), and there are several others to come.
Part of my challenge in writing Behind the Gloves is to present concepts and cultural values - especially those that are so foreign to Western readers - clearly and, as much as possible, without bias. It is very easy for authors, intentionally or otherwise, to present values that are "different" as "wrong," "evil," and so forth, especially when they are taken out of context and presented in a one-dimensional way. (Face it - you were taught to believe that "The Corps is Mother, the Corps is Father" means something NEFARIOUS.) To the extent that canon presents telepath culture with negative bias, I aim to walk it back and present values, beliefs and customs neutrally (or positively) and in proper cultural context.
Back to birthdays. Telepaths' "real" birthdays aren't a secret - some telepaths do inquire about them, or find out when they were born one way or the other. But it's not seen as a big deal, and some telepaths don't care or ever find out the exact day on which they were born. Bester, for instance, never inquired as to the exact day of his birth, because it didn't matter to him in the least. It didn't matter to his cadremate Brett, either, and at one point later in life they discuss how their not caring about such things is one of the cultural values that separates Cadre Primers (or more generally, Corps-raised telepaths) from laters, who began their lives in the normal world.
Counting age differently is, like gloves, a cultural point that separates telepaths from normals, and many Corps-raised telepaths like it that way.
Indeed, "individual" birthday parties would be seen in the Corps as selfish, since they involve the celebration of an individual over the community, and... not for any special achievement, really - just for being there. Corps-raised telepaths don't do that. Special achievements are honored (excellence in academics or sport, exceptional bravery or sacrifice, and so on), but "birthday" is collective.
Behind the Gloves uses the telepath age-reckoning system where it makes sense to do so (for example, in stories about Corps' raised telepaths such as Al Bester) and normal age-reckoning at other times (for example, in stories about laters who use the normal system).