is ten year old henry is bad at math?

okay so like i get that henry is literally 10 years old and went through the storybrooke school system...but like how did he believe his dad was a firefighter if he knew how old emma was, and how old she was when she had him??? literally he knows she must have been 17 when she was with his father which leaves two options of his father also being around the same age as emma or he's just a full grown adult sleeping with a 17 year old, which when i was 10/11 I knew that was bad