If they are brothers, I would guess Charming is older. Because, realistically, it isn't a that big an issue if the younger brother goes AWOL. I suspect you're right and they'll age HG up a bit and hope nobody does the maths!
update- found an article from December where they announced who was on the soundtrack including Sarah Brightman, Todrick Hall, Adam Lambert... do we really believe all these people are actually in the musical? That is the question...
I saw that on Amazon, before that removed it. IIRC, Sarah Brightman was only on the very first track, so I assumed she was doing a voiceover "Once upon a time, in the land of...".
Todrick Hall was only the last track, so I would guess a bonus/pop redo of a song from it.