How young is too young for the overnight sleepovers?

I have found parents of one child (“onlies”) are such HUGE fans of sleepovers from the get-go -(infancy) – since – hello! it is a free night for them. But for those of us idiots who decided to have more kids- — I don’t feel so compelled to have sleepovers happen until they are like 13 or so…..

??