Nah, it can't be age. There are some who are older and still gets roles. It's usually popularity I believe.
I say they both have their own looks. I don't think they both are the prettiest actresses, and are average, but that's just me.
Gypso's acting is not all that. She overacts sometimes haha. She got comfortable in her role and with push from eps 8 and on. For some reason, I visually like them together so it's nice to see them pair up again. This looks like the same production from their previous lakorn if I'm not mistaken.