As an adult who came back to the sport in her late teens, I'd say adults ought to wear whatever they want and feel comfortable wearing. For some it may be looser pants, others leggings, others skirts, and others yet dresses. Of course, for testing and competing purposes, a dress is preferable (says the girl who tested elements and skills in a skirt + form-fitting sweater until the higher levels). At that point, my opinion is to wear something age- and form-appropriate, although that holds for skaters of any age. If an adult is uncomfortable in a dress, then pants are fine. That's the joy of being an adult - we get a few more allowances than the children
However, a guide can be helpful, because some adults may not feel comfortable in short skirts, and it's a good idea to let potential newcomers know that that attire isn't a necessity. I started off in simple pants, and was fine with them until I caught my blade in my pant leg - I switched to a plain black skirt shortly thereafter. The reason for black wasn't that I wanted to be plain; I have many coloured sweaters and black matches pretty much anything
In that sense, I don't believe in imposing any kind of 'uniform' on any skater, adult or child. I do however believe that new skaters showing up for their first lesson in the whole attire (fancy sparkly dress, expensive brand sweater, expensive brand bag, etc.) is a little excessive; at the earlier levels, that's money better spent on lessons and ice-time, but I won't judge people for it, only wish they hadn't spent so much on things if the skater quits after a season.