I never co-slept with Emily either but she also goes through stages of coming in to us during the night - she'll do it a few nights in a row then just stop. Then a few weeks later the same thing.
I guess for me I'm quite happy to let her do this as long as she wants. I like it, she likes it, and because it's only occasional it's not really an issue for us. I dont' think there's any age to stop it if everyone's happy