A few months back I wrote some code to calculate age from a date of birth and arbitrary end date. It is not a real tricky task, but it is certainly one that comes up often when doing research on individuallevel data.
I was a bit surprised to only find bits and pieces of code and advice on how to best go about this task. After reading through some old Rhelp and Stack Overflow responses on various ways to do date math in R, this is the function I wrote ^{1}:


A few notes on proper usage and the choices I made in writing this function:
 The parameters
dob
andenddate
expect data that is already in one of the various classes that minimally inherits the base classDate
.  This function takes advantage of the way that R treats vectors, so both
dob
andenddate
can be a single or multielement vector. For exampleenddate
is a single date, as is the default, then the function will return a vector that calculates the difference betweendob
and that single date for each element indob
. Ifdob
andenddate
are both vectors with n>1, then the returned vector will contain the elementwise difference betweendob
andenddate
. When the vectors are of different sizes, the shorter vector will be repeated over until it reaches the same length as the longer vector. This is known as recycling, and it is the default behavior in R.  This function always returns an integer. Calculating age in years will never return, say, 26.2. Instead, it assumes that the correct behavior for age calculations is something like a
floor
function. For examle, the function will only return 27 ifenddate
is minimally your 27th birthday. Up until that day you are considered 26. The same is true for age in months.
This is probably the first custom function in almost 3 years using R that I wrote to be truly generalizable. I was inspired by three factors. First, this is a truly frequent task that I will have to apply to many data sets in the future that I don’t want to have to revisit. Second, a professional acquaintance, Jared Knowles, is putting together a CRAN package with various convenience functions for folks who are new to R and using it to analyze education data ^{2}. This seemed like an appropriate addition to that package, so I wanted to write it to that standard. In fact, it was my first (and to date, only) submitted and accepted pull request on Github. Third, it is a tiny, simple function so it was easy to wrap my head around and write it well. I will let you be the judge of my success or failure ^{3}.

I originally used
Sys.time()
not realizing there was aSys.Date()
function. Thanks to Jared Knowles for that edit in preparation for a CRAN check. ↩︎ 
Thanks to Matt’s Stats n Stuff for getting me to write this post. When I saw another age calculation function pop up on the rbloggers feed I immediately thought of this function. Matt pointed out that it was quite hard to Google for age calculations in R, lamenting that Google doesn’t meaningfully crawl Github where I linked to find my code. So this post is mostly about providing some help to less experience R folks who are frantically Googling as both Matt and I did when faced with this need. ↩︎