Are hobbies in the CV detrimental? I have a serious, artistic hobby and I write an application for a scientific job. The hobby is music. I studied music for three years, but didn't conclude, because I decided to pursue a scientific career. I kept playing horn1, however; since lately I play in a (non-student, serious) orchestra.  

I've seen that professors add information of this kind to their, say, online CV's. But first, they probably do not apply for grants using that version of their CV. Secondly, they are already somehow "respected". 

That hobby was an important part of my life, thus I'd feel somehow dishonest to exclude it from the CV. It also explains why I'm about two years older than an average PhD student is where I studied.

Is including an artistic hobby in my CV detrimental for my application? 

[1] This is not really the instrument but just an example.
It is part of your experience, so should go in. It also shows, to a degree, that there's more to your life than just science, making you appear more rounded. It also explains the age discrepancy. 

However, should the place you're applying for require a nerd, who would appear totally immersed in the task in hand,  it may be considered that you have a somewhat butterfly approach - trying out something, but not following it through.

Some thought will be needed considering the c.v., and be more mindful of the recipients of it. Thus tailor it to the exact post applied for: tell them what they need to know. Or what you think they need to know...