You are correct that vaccines are no longer offered for kids under 18, except if they have certain illnesses or conditions.
Vaccines including first booster is still offered and recommended for everyone above 18.
Second booster (i.e. fourth jab) only offered to people above 50.
Source: https://www.sst.dk/da/corona/Vaccination
Google translate:
https://www-sst-dk.translate.goog/da/corona/Vaccination?_x_t...
Quote:
> 10. Do you still recommend that one take the 1st, 2nd and 3rd shots against covid-19 if one has not yet been vaccinated?
> The Danish Health Authority continues to recommend that persons who have not been vaccinated be vaccinated with the 1st and 2nd shots. This also applies even if they are younger than 50, as there may be a certain risk of serious illness if you are not vaccinated.