Any idea what is MC doing? Studying ? Where ?
And any announcement about Christian's future steps? Studying ? Where ? Or military ?
Christian is almost certainly going to sign up for volunteer conscription. (At present almost 100 % of all conscripts are volunteers, but it happens some are drafted, especially if they pick a very low number at the ballot.) That means boot camp, after which he is will go to NCO school, followed by the officers academy (unless of course he is totally unsuited for the military, but he seems healthy, fit and of sound mind) so when he is around 21-22 he will get an audience with QMII as a newly commissioned first lieutenant. After that he may spend an additional year or so in the military.
After that I imagine he will go for a gap year and travel somewhere. Australia springs to mind.
Then it's off to university. He may not study for a bachelor but attend a number of all-round courses suited for his needs and interests - and talents.
Political science, communication and history seem obvious to me. And no doubt something else as well.
He will have an advantage when he starts at university. He is likely to be a little older than average. He has completed a good leadership course with experience, which means he will be used to dealing with all sorts of people, used to having a responsibility and used to self-discipline.
Aarhus University seems obvious, because both Frederik and QMII studied there and have very fond memories of that place and it means that he won't be "moving home" again. - Once you have been a conscript, you are not moving back home.
I should be extremely surprised if Christian is not studying for several semesters at foreign universities. Again an Australian university seems obvious. (Not least after Nikolai has been there and hopefully has fond memories of that.) A top British and/or US university also seems obvious.
Had Joachim been crown prince, I'd say Sorbonne would be a very obvious choice as well.
And if things really should turn out to be serious between Christian and MC, perhaps an Italian university?
After this basic "future heir course" at university, he may opt for another gap or an extended travel somewhere, like Frederik did, before returning to the military and earning his third diamond and then first star, as captain and then major. By then he should be in his late twenties.