If you look at what makes up the best coordinators, it's their ability to strategically build gameplans and gameday play calling that sets the greatest ones apart. Choosing the best from among them for a head coaching position comes down to how well they lead, connect with players, etc., so that you will get the guy who can grow into that role as the central figure for the organization.
The focus on McVay's age is really the wrong way to approach it. Here's the questions that concern me:
How well does he handle alpha males on his staff and will he be able to bring in guys who aren't just his buddies? In McVay's case he is a guy who not only survived a house cleaning, but he was promoted the next year and has seized the offense under Gruden while leading veteran coaches like Callahan. This is not an element that concerns me as if he is a top option for the Rams right now it means his staff list was not a collection of unknowns and buddies.
How well does he translate to players in his ability to communicate and get buy in? Players love the guy and won't shut up about him. He's not some douchenozzle personality like Haley who players want to choke, but he's also not their friend. The dude is in charge and you can see it in his interactions that have been posted up here on vids.
How well can he handle the media and endless details that pop up? I see no issues here. This guy has no family, he is married to the game. Now that might make him a risk of burning out like Vermiel did, because he's probably going to sleep in his office at first. But that's a concern for later.
Will he be able to toe that hard line when he needs to with staff and/or players? This is the risk with him. But honestly when I look at guys like McDaniels, who struggled with all three of these in his Denver experience, I think it's a risk inherent with any young coach. This is what I'm talking about when I say they need to grow him into that head coach they want.
Demoff should have a real good idea through his Dad of the Rams chances of getting Shanahan, so I don't have an issue with them waiting to interview him. But personally I'd just lock McVay up now and start building it around him, give him that edge to start hiring his coaches immediately.