Returns the age-interpolated median of the Dutch references (van Buuren 2014).
The working range is 0-3 years. This function should
be called when the
key = "dutch".
Decimal age, numeric vector
A vector of length
length(t) with the median of the Dutch references.
Internal function. Called by
dscore:::count_mu_dutch(0:2) #>  4.88220 48.90290 63.83718