I actually can make a decent case for why they should be 20 and 21! Without spoiling you, even!
When you first meet Cecil, he talks about how he was adopted by the King and raised by him (implication: as the heir to Baron.) Kain, same deal. Presumably they have both spent their entire lives in intensive training to be important men and, therefore, would be very well-educated and trained in the military, since Baron's is a militaristic culture. In my headcanon, most young men in Baron go into the military at sixteen; so they would have had four years apiece in the military of a country whose entire mindset revolves around the military. Add this to the fact that they were intended to inherit the kingdom, and they would be very mature and competent.
(Please forgive me; I can tl;dr meta about this game for DAYS.)
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When you first meet Cecil, he talks about how he was adopted by the King and raised by him (implication: as the heir to Baron.) Kain, same deal. Presumably they have both spent their entire lives in intensive training to be important men and, therefore, would be very well-educated and trained in the military, since Baron's is a militaristic culture. In my headcanon, most young men in Baron go into the military at sixteen; so they would have had four years apiece in the military of a country whose entire mindset revolves around the military. Add this to the fact that they were intended to inherit the kingdom, and they would be very mature and competent.
(Please forgive me; I can tl;dr meta about this game for DAYS.)