It's against the law
I am 18, and recently I have been getting to know a girl in my street. We get on really well, and I think that she feels for me more than just a friend. I also think that I am starting to develop feelings for her too.
The problem is that she is 12 years old. Would it be wrong to start a relationship between us? Obviously non-sexual, but I feel that we need to take it further. Should I tell her, or should I keep my feelings out of our friendship?
You don't actually say whether you are male or female.
My advice to you is to remember that people on your street are your neighbours and should be treated as such. neighbours watch everything that goes on in a street and This 12 year old is a child of one of your neighbours, and it would be wrong, illegal, and most importantly against the law to pursue any kind of relationship with this child she is a minor remember this.
You are an adult and should be making friends/ relationships with other adults and not children.
This child probably has a crush on you, (everyone has crushes) and this should not be taken advantaged of because she is a minor. Keep your distance, and let her play with her friends of her own age. In time her crush will fade away.
Try to avoid her or look the other way, don't stop to speak to her, or encourage her this may hurt her feelings, but you are a responsible adult and should act like one!