So your co-worker generalizes from his (lack of) experience, and predicts that everyone else will have a problem. How very human of him. I often catch myself doing the same.

When coding, do you assume a certain level of expertise of the lucky so-an-so who has to maintain the code?

I assume that they'll be a notch or two behind my abilities, and try to write code in a way that will help them up the learning curve. This means descriptions of any non-obvious data structures, and no tricks unless well commented.

Give people good examples to work from, and they may surprise you by how quickly they catch on.