Perfect constructions speak about a state which obtains at Reference Time—the time which your discourse addresses—as a result of some prior event. Present perfect and past perfect address different Reference Times.
Present perfect speaks about a present Reference Time:
I am about to start my first novel, but I have been writing since childhood. ... This speaks about my present writing experience.
Past perfect speaks about a past Reference Time:
In 2003 I started my first novel, but I had been writing since childhood. ... This speaks about my writing experience in 2003.
You may read more (much more, possibly, than you want) about the meaning and use of perfect constructions here.