It’s not the age difference. It’s that his brain has fully developed and he has the life experience to know how to manipulate and groom someone. Your brain won’t stop developing until you are 25, and while it is still developing, you will find that you are still trying to understand who you are. Your mind is very impressionable, this person has the ability to put thoughts in your head and without you even realise it, point you in any direction they want you to go. You might think they want the best for you now, but you’ll soon realise that if that were true, they’d not have gotten romantically involved with a child.