Thiago is way ahead of the others. He started playing for the first team in 2009, signed officially in 2011. Montoya, Cuenca and Tello debuted in 2011 (two years later) and signed in 2012, and Deulofeu is obviously still a Barca B regular. I don't think age has that much to do with it, honestly. Thiago debuted in 2009 because he was obviously good enough at that point. I'd definitely separate him from the others, in terms of potential, talent, current progress and whatnot. He's no longer a current prospect, in comparison to the others, he's almost a regular. But between Montoya, Cuenca and Tello, I'd definitely say Montoya is the one who's currently the most reliable and mature as a first team player. I wouldn't argue about which one of them has the most future potential, especially when you include Deulofeu, but as far as current progress goes, if I had to say which one of them I'd trust most to do their job well, I'd put my money on Montoya ahead of the rest every time.
But I agree with what you said about not many great defenders being around at this time. But then again, that's another reason why I believe Montoya deserves more of the "spotlight" (for the lack of a better term) comparing to all these wingers La Masia is producing lately.