What called is when you are 15 years old and attracted to 17 years old?

It's called being horny.

It's normal and natural and healthy.

It's an adult biological trait.

I would say it is called normal teen behavior called Letcher.

It seems totally normal to me.

A Pedophile lol 

A hard on . . . .

its called a joke that nobody else takes seriously 

It's called puppy love, so grow up and smell the flowers.

If I was attracted to teenagers, it would be ephebophilia.  When teenagers are attracted to teenagers, it's called normal.

Such a person is called a "hirquiticke." That's the technical term. It derives from a Greek term meaning older than 14 and refers to profound sexual attraction between teenagers over age 14 for each other, a "hirquiticke" being such a teenager. What you refer to, an "ephibophile," is a term for an older person, a person no longer a teenager, who is sexually attracted to or fetishizes teenagers in their mid- to late teens.

Freud might psychoanalyze that an ephibophile is someone who, likely due to some form of emotional trauma suffered while a hirquiticke, became developmentally stunted, thus not outgrowing it, as a hirquiticke normally does, but instead remaining psychosexually fixed at that stage in their development, resulting in an ongoing sexual fixation for teenagers of the age hirquiticke's are attracted to and fetishizing them long after no longer being a hirquiticke.

It's called a hard on