I thought their first meeting, especially once FW has left the scene, had meet-cute type of chemistry, but subsequently there was more of an older friend and mentor vibe, true. Re: Fritz' actor - who btw is the greatgrandson of Käthe Kollwitz, legendary Weimar artist - true, he does look young, especially in the execution sequence, but then after the childhood sequence we do skip to a point where Fritz is 15, and as you say, the actor was 20 at the time, i.e. fitting with Fritz' age when the story ends. He does look more mature in his scenes with both the ambassadors (when he cryptically say "oh, I have plans alright" (for when he's king) and with Doris Ritter (not just because the actress plays her as shy while Fritz is confident but because of the way his hair is pulled back and the face is lighted (from below, not above), plus he's in "civilian" clothing whereas in his scenes with FW he's in uniform 99% of the time, which in addition to the difference in physical mass emphasizes his youth. Mind you: given Fritz was young, really, really young, it's fitting.

Re: Jan Niklas being 13 years older - he played young Peter the Great (from 18 to his early 30s, then Maximilian Schell took over) six years after playing Katte in "Der Thronfolger", so presumably tv producers thought he was looking youthful enough when casting him and only later once the actors actually played together thought they just didn't look like within the same age group?