This fight isn't technically licensed pro boxing is it? I'm assuming that the rules for exhibition stuff like this versus official pro-boxing are different in terms of concussion protocol stuff. If he really did get knocked out twice by his sparing partner, one, he probably shouldn't be fighting period, and two..that's some scary concussion stuff to have to worry about. However I guess at the end of the day all these guys know they are taking huge risk for having mush for brains in their latter years.
It's a pro bout. Silva has also recently been fighting as a pro and won both his fights last year, the last being against Tito Ortiz so I don't think you have to worry about Silva. As for getting knocked out in sparring I'm sure that even happens to the best of the best but they probably usually keep quiet about it if so. Not the best of looks to admit to it but maybe he did misspeak or he and his camp are just backtracking.
What fighters? There's really not that many and I don't think there's any in the UFC that are that age or even near it. Once you're pushing 40 you just can't keep up with the kids no matter how great you are and Silva is nearer 50. MMA is just too taxing on the body to competitively compete at that age, hence why a lot go to boxing at the tail end of their careers.