I'd see "I can't..but I can" as saying "what you see is this, but what I am is something else". Which is more or less the same as Si Kahn is saying in that song - up to that last verse it's a great song too. Which shows that even great songs can use a bit of constructive criticism.

I imagine we'll likely have a chance to follow Harry Potter into adult life. Ursula Leguin has already done that with her student wizard Ged in A Wizard of Earhsea and the later books. (And there aren't any children in the Lord of the Rings. And Tolkien was very insistant that the idea that stories like that weren't for childen as such was nonsense. Or young adults for that matter.)

There are supposed to be sea birds that can never land - whether it's striucly true I wouldn't know, but it's a powerful image that ties into that line of yours. Just as mermaids could swim but not walk, and sing but not talk.