No Agenda, It is all a matter of context. The system of tenses in English is very flexible. It oftens has more to do with your mental perceptions of time, than an actul timeline. We are always living in the present moment.You should avoid drawing conclusions from a sentence isolated from any context. The statement could just as well come from someone who is talking about the fact that they knew someone even when he was a child. For example: I have been married for 25 years. I knew my my wife even when I was a child. We are destined to be together forever.

Yes, I think that this sentence could imply that the speaker knew someone when they were a child, but they have los contact. However, if someone says, "I have known him since I was a child," I would assume that they met the person when they were young and they are still in contact with that person. I hope that helps. God bless. Have a wonderful day.