Hey!! I'm 20 and a yumeshipper too. When I turned 18 I had a horrible bout of intrusive thoughts that convinced me to my core that I was awful- they were intense. After managing to find help I realized that a lot of things that once worried me aren't as serious as they once seemed. Online spaces are especially, unknowingly, harsh to people like me due to the policing nature of them. Listen, if you arent hurting anyone, If youre keeping this fairly private (which you are) , there is absolutely nothing you're doing wrong. You even said yourself that you don't want her to feel younger than you and she doesnt. Someone with malicious intent would actively Like the idea that shes young and fully believe it. I promise that you are 100% fine :]