The speaker is remembering someone they lost when they were very young, probably a person close to them, though the poem never says who or how. The opening line sets up a painful gap: the speaker was too young to remember much, yet old enough to understand what was happening. That gap drives everything that follows.
The idea that our cells are replaced every seven years becomes a way of describing how loss physically fades. The touch, the voice, even the happy memories are slipping away, and what remains is mostly pain. The speaker admits to hiding from that pain as a child, and now feels regret, perhaps for not holding on harder while the memories were still clear.
The mood is quiet, sad, and a little guilty. The short, plain lines feel like someone admitting something they've kept inside. The ending, with only old photographs left, is bleak: the person has become an image rather than a presence.