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The general idea is that a lifelong friend is one who has supported you, shared with you, listened to you, and encouraged you throughout your life. This is particularly true when you have a person who is willing to stand by you, altruistically, during life-changing events that affect you directly.
Some people may say that a lifelong friend is one who was raised in the vicinity where you were raised and has interacted with you throughout most of your lifetime. However, that does not seem to be a very appropriate description because physical proximity does not entail support nor in-depth interaction.
The best descriptor of a lifelong friend would be the person or persons who have intervened throughout your life as as your direct social support system. Since most individuals need a support system in place to aid them through the many "crazies" of life, you can certainly call those who are there for you your "lifelong friends."