Returns the age-interpolated median of the D-score of the default reference for a given key.

`get_mu(t, key, prior_mean_NA = NA_real_)`

- t
Decimal age, numeric vector

- key
Character, key of the reference population

- prior_mean_NA
Numeric, prior mean when age is missing

A vector of length `length(t)`

with the median of the default reference
population for the key.

Use `get_reference()`

for more options.