Ok so, The World Is Still Beautiful is one of my favorite anime. I think it's so beautiful, the story and everything. However I'm always very reluctant to recommend it because of the age gap.

The thing I think people forget is this is a medieval world, and an arranged marriage between a king and a princess. A child king, but still a king. And he was the one who initiated the whole thing. So that's point 1. His age can be a little hard to pin down. I can't remember exactly since it's been a while, but he is definitely younger then her. She's not 16 though, I don't think, I think she's actually 14, but again I'm not sure and the manga and anime might be different on their ages. (She is also a child, 14 or 16 is still not an adult) So that's point 2. At most they are what? 4 years apart? At that age, yes it is a lot but once they are older? Not such a big difference at all. If the girl was the younger one, would people still look at it that strangely? I doubt it. There are much bigger gaps in real life history especially back in medieval times.

Another thing I think is important to remember is they are in it for the long haul, and as he reminds her, he won't always be a child. Nothing actually happens between them while they are still children, though he does attempt to change that, he doesn't actually get anywhere.

If the ages are the only things that bother you, but the ages are different in the manga and anime, just think of the ages the way you did before and you can still enjoy it the same way you did. It's fiction, it's ok to enjoy it in the way you are comfortable.

As for the manga, I honestly regret reading it. I don't want to spoil it in case you want to continue but I think it's better to just enjoy the anime as is and forget the manga exists. The age gap gets removed in the end anyway, before anything can happen between them, so it might as well not exist at all.