Totally agree with your point about floyd being old. But I'm not convinced floyd isn't going to fight in the UFC. 40? Hell no you can't start at 40 and do well. 30....at least we've seen that. But I think even at 30 he wouldn't do too well with 2 years of training. I mean 30 is old especially for that weight class. Not to mention you're 32-33 when you take your first fight.
What I think it boils down to is the preceding style. Like DC, Romero, Lesnar (all got a very late start in mma) but they were all elite wrestlers. That's going to buy you a lot of time in MMA because you have a greater chance of being able to dictate the fight and land takedowns. Plain boxing isn't stopping nobody who's at brown belt level bjj or even just a decent wrestler.