LeoPalace may have rules concerning this, but I doubt there is any law directly restricting the age at which one can rent an apartment. I imagine it would be surrounding laws that would affect this, such as negligence on the part of parents for leaving a minor alone and stuff like that.
I know its not a clear answer, but I say it anyway, because I think you won't get a clear answer, simply because I think there is no such law.
I think that want LeoPalace is going to be most concerned about is getting their money. So you may need to provide a guarantor or two that they trust for being employed legal adults.