This is my first question on this site. I am not a native speaker. My question is, is it formal or informal to use y/o as an abbreviation of "years old" in British English?
@Edwinashworth and @Dan Bron RE discussion in register. I think that there are multiple definitions of register. Simplified definitions focus on "informal" and "formal" registers, however, these fail to formally identify what linguistic features realise the register variables. As an ESL teacher, I find many textbooks over-simplify this issue, resulting in students learning lists of "informal" and "formal" words e.g. get = receive, buy = purchase, and = furthermore etc. Although there may be some contexts where this is possible, this is simply not the case.
As a student of Halliday's systemic functional linguistics, the term "register" is further technicalised to include notions of what's going on (the field of discourse), who's involved (the tenor) and how these are textually linked together (the mode). However, even within SFL, there is still much discussion about what this actually means and there are different camps with different definitions. One camp believes there are registers for domains of use e.g instructional, regulatory, representational, etc. Another camp believes that register relates to the context of situation e.g. medical, legal, educational etc. The reason I bring this is up is that just last night I was discussing differences between Halliday and Martin's understanding of register (see http://functionallinguistics.springeropen.com/articles/10.1186/2196-419X-1-3)
I have been schooled in Martin's view of register and understand it to be a complex interplay between the field, tenor and mode, each of which can be related to semantic systems, which in turn can be related to lexico-grammatical systems. E.g. Field can be related to whether the meanings are more common or specialised/technicalised (e.g. dog - canine), tenor can be related to whether the relationship between interactants is more close or distant (e.g friends and strangers) and whether the power relations are more equal or unequal (e.g. student - teacher), and mode can be related to whether the message is more spoken or written (remembering that written language can be spoken!). From this I understand "formal" to be a combination of more specialised/technical vocabulary (field), greater distance and power differential between interactants (tenor), and tends to be more written language (mode). "Informal" can be seen as a combination of more common/everyday vocabulary, closer distance and more equal power relations, and tends be more spoken. As such, jargon may be classified as a form of specialised language, which may in turn suggest a particular register, but the relations between the language and the context must be made explicit. Personally, I find it difficult to answer whether years old is formal and y/o is informal.
I hope this response is not seen as a definitive answer, but rather one that encourages us to consider how terms such as "formal/informal" may in fact be forcing us to engage with false dichotomies. I believe there is much more to be done to clarify our understanding of register and welcome any further discussion.