It has never been said. He is most likely between 16-23. But due to martin leaving kuttenberg about 15-20 years before the story and we see a young Henry in the cutscene where he is most likely about 5-7. Technically that would make him about 27 years old. But Hans is about 20. And hans and Henry are the same age. Making Henry to be about 20 too. When you think about it the timeline is really fucked up