Are 15 year old girls on the same level of maturity as boys their own age explain your answer

Some are, and some aren't. Its's more of a personal thing. 

Really it depends on a lot of different factors, such as the type of friends the individual hangs out with. For example, lets say you are a 15 year old boy who hangs out with a bunch of pranksters and people that get in trouble a lot. And there is a 15 year old girl that is in the same grade and goes to the same school is very focused on grades, she does a sport and has a job. SHe doesn't get in trouble and learns from her mistakes, because she has made solid friendships. Who do you think is more mature? The girl or boy? It also depends your view of what mature is.

Is mature to you, somebody who can do grown up things and no longer cares about "having fun."

Is it somebody who knows how to have fun but can still make good choices.

Another thing to consider is the situation at home or school. Example would be what if somebody was home schooled there whole life, would they be more mature than the people in who grew up going to public school? 

I hope this information helps!