Your friends are people you know well and like spending time with. You can refer to a friend who you know very well as a good friend or a close friend .
A close friend told me about it.
If someone has been your friend for a long time, you can refer to them as an old friend. He or she is not necessarily an old person.
"A friend should bear his friend's infirmities" [William Shakespeare Julius Caesar ]
"The belongings of friends are common" [Aristotle]
"My best friend is the man who in wishing me well wishes it for my sake" [Aristotle Nicomachean Ethics ]
"Friends are born, not made" [Henry
Adams The Education of Henry Adams ]
"Consists not in the multitude of friends,"
"But in the worth and choice" [Ben Jonson Cynthia's Revels ]
"Friends are God's apology for relatives" [Hugh Kingsmill]
"Old friends are the best. King James used to call for his old shoes; for they were easiest for his feet" [John Seldon Table Talk ]
"Old friends are the blessing of one's later years - half a word conveys one's meaning" [Horace Walpole]
"The only way to have a friend is to be one" [Ralph Waldo Emerson Essays: First Series ]
"Of two close friends, one is always the slave of the other" [Mikhail Lermontov A Hero of Our Time ]