Is it illegal to date a 19 year old if you are 15 in Idaho?

There are no laws on "dating" You may date a 15 year old, as long as the parents of the 15 year old are OK with it and give their permission.

The law is about having sex, that would be illegal.

When using a site like this, its is a good idea to search around first for similar questions. Had you done so, you would have found many similar questions and learned that there are no laws against dating.

However a minor (under 18) is still under parental control. So they would have to approve the relationship.

The age of consent in Idaho is 18, so any sexual activity could be prosecuted as rape.