What exactly does a photographer's assistant do?

I'm fifteen and I have been doing photography for a year and a half now. I use a Nikon D3300 and use Adobe Photoshop as a software. I post my pictures on Facebook and a couple weeks ago my moms cousin asked if I wanted to be his assistant. I get paid like 50 bucks a session so it's pretty good money for my age but I'm not sure if I want to do it or not.

Depends on what sort of photography the photographers does. But go for it, get the experience. Whatever you are asked to do it will be valuable experience in life and in the world of work, even if not completely relevant to your style of photography.

Many years ago, when I was about 20 years of age, I assisted in a theatrical, advertising and fashion studio in London. Great fun (all those gorgeous models) and great experience. I once even got chucked out of a London West End theatre by a leading British actor (Sir Lawrence Olivier, as it happens).

The care and feeding of the camera gear. A great way to learn the craft. But beware, a job with a lot of responsibility. A bad assistant can ruin a shoot if something is spoiled. And if an assistant makes a mistake - never hide it! Tell the lead photograher as soon as possible - maybe additional shots can be made to save the assignment.

You'll probably be holding reflectors and lights at first. It also depends on his subject (portrait, event, studio). I'd say do it, you learn a lot, and get spending cash!