My six-year-old younger daughter still sleeps with us from time to time at the early morning hours.
She gets to sleep in her own bed each and every evening (with the exception of some rare cases, like when I am on a business trip away from home for a few nights). She sleeps very well through most of the nights and wakes up happy in her very own bed.
But from time to time (I'd say once per a month or once per 2-3 weeks) she gets up some time between 3 AM till 5 AM, grabs her pillow, "checks in" to our bed and immediately falls asleep as if it was as normal a situation as can be in the world. No arguing, no asking questions, no nothing. Sometimes she does this as silently as possible and I simply wake up in the morning, finding her next to me, to my extreme surprise.
(I'm a bit worried about her security due to my oversized body and high weight, so most of the time, when I'm aware that she came to us, I'm packing myself and I'm going to the living room to sleep.)
Her older sister (now 9) is not doing such things at all (again, except for my business trips, when both my daughters sleep with my wife for an entire night) and was not doing so when she was at her younger sister's age (5-6 years). I don't quite recall this now, but I think that she stopped coming to our bed around being 4 years old.
Shall I be worried with this situation, by any means? Is there any "scientifically proven" age when such behavior should stop? Or when it should become a concern for parents?
I recall a book that I read some time ago (around eight years ago, when her older sister was around 1 and younger one was not with us yet) which possibly claimed that an over 2 years old baby, still sleeping with their parents, shall be a concert. But, I must underline that since it was around 8 years ago, I might have forgotton or messed up things that I read.
My wife claims that this not only isn't a problem, but it is in fact a good blessing for us. That I should stop worrying or even thinking about this and cherish each of such nights, because it might not last for long (giving our older daughter as an example).