Can I go to a festival when I’m 15?

It won’t be. From the corresponding Rules of the organizer:

“Youth from 14 to 16 years:

Young people aged 14 to 16 are granted admission only with the assistance of a person entitled to care or a person entitled to education. The guardian must be at least 18 years old, must present an identity card and a written instruction (so-called ‘Muttizettel’). The written education order must be completed and signed by a person entitled to care.

And further down:

What does a “education person” mean?

An educational officer is a full-year person who performs certain educational tasks in time-limited manner on behalf of and in place of the persons entitled to care (i.e. the parents) (e.g. accompaniment / supervision). In the case of an accompaniment by an educational officer, a written education commission is required, so-called. ‘Muttizettel’ is required. The “Muttizettel” is adjusted to the accompanying person’s existing identity card and checked for completeness. Where appropriate, the person entitled to education must provide a copy of the identity card of the person entitled to care.

The Hype Festival is here: https://www.hypefestival.de/_files/ugd/05e1a0_896be3f5047248f38a38c7e6c4ff40c9.pdf

However, you can easily find out all this yourself if you just have a look at the organizer’s website.