My son's "40 grandpa" hung around for several years (started at age 2... strong presence until about age 4, checked in from time to time from 5-6). His concept of his 40 Grandpa is that I'm 1. My mum is 2. Her mum is 3. Her mum's mum's is 4. etc... all the way out to a 40Grandmother.
At around age 4 his 40 grandpa introduced him to Sing Tsank, who was my son's best friend a long time ago in china, but who was now a small child (about age 2) that needed my son to take care of him / play with him/ love on him until he died. ((Concept being that Sing Sank was actually a very old man still living in China, but that his mind was "walk about" / aka alzeheimers type... so kiddo was supposed to care for the man's spirit until the man finally let his spirit go)). Sing Tsank was never a constant presence but would show up for a few weeks, be gone for a few weeks, show up for a few weeks, etc. When my son was 6 going on 7 he cried for 3 days because Sing Tsank finally "died".
My son is now 8, and while he remembers both Sing Tsank & his 40 grandpa... it's nebulous. Even though this kid has a memory like an elephant and can recall quite easily events from about 18mo onward. They're "gone".
I'll treasure kiddo's 40 Grandpa and Sing Tsank for the rest of my life.
((ROFL... I just popped up with a memory of Sing Tsank, actually. I was "pushing him on the swing" at one point for kiddo, and as expected the empty swing was kind of wildly swinging. "Mom!" kiddo interjected "Spirits don't weigh anything, he's not going to slow the swing down... push SLOWER."))
My son has ALWAYS been a raging extrovert, and very, very social. There is NO way I could provide him with all the social interaction he craves. It takes him about a week of being with 10+ kids 24/7 for him to seek out small periods of time alone (from actual experience). He is just NOT a "play by yourself" kind of guy.
I remember being kind of worried about his 40 grandpa in the beginning. But, come to find, he gave pretty good advice to kiddo... "Mom, my 40 grandpa says I should come apologize. *I* don't think I should apologize, but he says I might have hurt your feelings." ... "Mom... 40 Grandpa says that we all get scared, but we need to figure out if we're scared for the right reasons. Is that right? What are the "right" reasons to be scared? He wouldn't tell me. He said to ask you." Miss that man, actually. Quite a bit.