What you say is interesting. If you define lifetime as the run from conception to physical death, it implies that Christ could not be for eternity with God because, during his time on Earth, he must have been in the process of growing. But that's obviously not the case. He was what he is at all times. During his baptism in Jordan it just became crystal clear to him who he is and what is his true nature and, mind you, his mission wasn't clear to him only after his famous stay in desert. If that was not possible for everyone, why would Jesus say that we can all be like him? (and some could even outperform him in terms of miracles – but that's only an inconsequential cherry on the top). He didn't lie.

So, I take it that by lifetime you mean a process that ends with death of separation from God and which does not necessarily converge with physical death (there are cases when it does, though). That would sound about right to me. You can be in Heaven and physically alive to tell the story. You can be “in eternity with God” while being a human child at the same time.

Where it goes after physical death of human body, I consider a big unknown. On the other hand, it will always be “in eternity with God” whether I'll be aware of it or not.