I've Kinda be one to go with the nobodies not aging. There is no evidence either way and as a leap of faith I think it makes for a better story that way.
And with the yellow eyes, I was under the impression it was a side effect of using the corridors of darkness. I think Lea and Isa's fall to nobodies had to be quite different from the rest of the experience. In my mind with the breaking up of radiant garden they fell in with Xehanort's group and things led to another. I don't know what else to say at this moment. I got a lot of theory of how it works but nothings confirmed so I am no better than anyone els.e
That's not really a possibility, all of the nobodies use corridors of darkness(except Roxas whose attribute is light). The only nobodies with golden eyes are Xigbar and Saix.
To me, so far out of all the Saix theories the one 'Saix was experimented on by Xehanort' is the most believable and makes the most sense. There's no doubt in my mind he and Isa are connected to Xehanort.
As for Lea, I think Isa persuaded him to become a nobody. Saix clearly states in KH2+FM that Axel couldn't stand not having a heart, which makes me believe Lea didn't jump into the darkness 100% willingly like the apprentices. Maybe becoming nobodies was a part of their 'plan' all along?