Imaginary friends

I was just wondering if I am on the right track with Fay. She is 2 and a half and has a really good friend at preschool called josh. He is now 5 and started school about 6 weeks ago. Fay really misses him and talks about him all the time. They have met up a few times since, socially and Josh's mum is a teacher at the preschool so he comes in some times. They play so nicely together and he is so caring of Fay and really bears in mind that she is much younger and "protects" her from a lot of rough and tumble from his older friends. He has been a really good influence on her as well as she does a lot of big girl things now "like Josh"
Thing is she has started playing with him at home, when he is not here, often having tea parties or playing in the bath with him, that sort of thing, she fell over the other day and Josh apparently pushed her.

Just wondered if it was just that she was missing him or whether we were entering the realms of imaginary friends. I think it's really cool that this little boy has caught her imagination and made such a striong impression on her and I'm not worried if we have an extra friend in the house. Just wondered what sort of age this behaviour started, maybe I just think cos Fay will be an only if I just think she is too young for everything

any thoughts or experiences would be welcome