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reference/age.md

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+# Age in Years of Individuals
+
+Calculates age in years based on a reference date, which is the system
+date at default.
+
+## Usage
+
+``` r
+age(x, reference = Sys.Date(), exact = FALSE, na.rm = FALSE, ...)
+```
+
+## Arguments
+
+- x:
+
+  Date(s), [character](https://rdrr.io/r/base/character.html) (vectors)
+  will be coerced with
+  [`as.POSIXlt()`](https://rdrr.io/r/base/as.POSIXlt.html).
+
+- reference:
+
+  Reference date(s) (default is today),
+  [character](https://rdrr.io/r/base/character.html) (vectors) will be
+  coerced with [`as.POSIXlt()`](https://rdrr.io/r/base/as.POSIXlt.html).
+
+- exact:
+
+  A [logical](https://rdrr.io/r/base/logical.html) to indicate whether
+  age calculation should be exact, i.e. with decimals. It divides the
+  number of days of
+  [year-to-date](https://en.wikipedia.org/wiki/Year-to-date) (YTD) of
+  `x` by the number of days in the year of `reference` (either 365 or
+  366).
+
+- na.rm:
+
+  A [logical](https://rdrr.io/r/base/logical.html) to indicate whether
+  missing values should be removed.
+
+- ...:
+
+  Arguments passed on to
+  [`as.POSIXlt()`](https://rdrr.io/r/base/as.POSIXlt.html), such as
+  `origin`.
+
+## Value
+
+An [integer](https://rdrr.io/r/base/integer.html) (no decimals) if
+`exact = FALSE`, a [double](https://rdrr.io/r/base/double.html) (with
+decimals) otherwise
+
+## Details
+
+Ages below 0 will be returned as `NA` with a warning. Ages above 120
+will only give a warning.
+
+This function vectorises over both `x` and `reference`, meaning that
+either can have a length of 1 while the other argument has a larger
+length.
+
+## See also
+
+To split ages into groups, use the
+[`age_groups()`](https://amr-for-r.org/reference/age_groups.md)
+function.
+
+## Examples
+
+``` r
+# 10 random pre-Y2K birth dates
+df <- data.frame(birth_date = as.Date("2000-01-01") - runif(10) * 25000)
+
+# add ages
+df$age <- age(df$birth_date)
+
+# add exact ages
+df$age_exact <- age(df$birth_date, exact = TRUE)
+
+# add age at millenium switch
+df$age_at_y2k <- age(df$birth_date, "2000-01-01")
+
+df
+#>    birth_date age age_exact age_at_y2k
+#> 1  1980-02-27  45  45.73973         19
+#> 2  1953-07-26  72  72.33151         46
+#> 3  1949-09-02  76  76.22740         50
+#> 4  1986-08-03  39  39.30959         13
+#> 5  1932-11-19  93  93.01370         67
+#> 6  1949-03-30  76  76.65479         50
+#> 7  1996-06-23  29  29.42192          3
+#> 8  1963-09-16  62  62.18904         36
+#> 9  1952-05-16  73  73.52603         47
+#> 10 1952-11-14  73  73.02740         47
+```