The very earliest memory I have of seeing a guardian spirit or angel was when I was about 3 years old. In the church nursery of all places.
A group of us were playing with a wooden rocker. By turning it over, it became stairs that we could run over. Just as the adults watching us were praising our inventiveness and resourcefulness, one of the other kids tripped and fell. Immediately, a spirit was there and guided his head so that he didn't crack it open; in fact, he wasn't really hurt at all. What could have been a concussion was just a near miss.
I saw the spirit. My friend saw the spirit. The boy who fell did not. The adults did not though they did offer prayers of gratitude that the boy didn't hurt himself. While the rest of the room focused on the boy that fell, my friend and I watched the angel. S/He smiled at us and faded away. In the serious voice that only children seem to have, we talked about seeing the spirit with the boy that fell. He confessed that he used to see them but didn't now that he was growing up.
The spirit was definitely not a ghost. The aura around him/her was different. In retrospect, I believe the spirit assumed a humanoid form so that it would seem 'right' to us -- we knew about angels after all. There was definitely a sense of peace from the spirit. But I still have unanswered questions about this encounter, so many years later.