Something that is "lifelong" lasts for as long as you're alive, or almost that long. Here are some common phrases that use the word "lifelong":

• a lifelong friend (someone who's your friend from a young age until you both grow old)
• a lifelong dream (something that you've wanted to do since you were a child)
• a lifelong commitment (a promise to do something, like be married, for the rest of your life)
• lifelong employment (working for the same company for one's whole career)