Birthdays, for those who celebrate them on Krynn, are called their Day of Life-Gift. Kang has no idea what day he hatched, just like all first generation males; all he knows is that it was somewhere around late spring/early summer of 342 AC. Krynn also has a slightly shorter year with only 336 days, soooo he's roughly 45 on Krynn and 41 on Earth.

That's just his chronological age. Draconians can live up to 1,000 years (though they don't know it yet), and the first generation (which he's a part of) were adults by about two years. Natural generations are adults by around 18-20 years. He's the equivalent of about 22 or 23, compared to humans.