Something lifelong lasts your entire life, or for most of it. A lifelong friendship might have begun in kindergarten, and it continues today.
You can describe your grandfather as a lifelong Republican if he's voted for the Republican candidate in every election all through his life. You could also say you have a lifelong hatred of green vegetables if you've refused broccoli and spinach since you first ate solid food. The word lifelong, meaning "continuing for a lifetime," was coined in the nineteenth century.