I think it's more affected by the individual player than a player's age. Also, a lot of injuries from collisions/contact would happen to any player of any age. Off the top of my head, I think most of Kamara's and Hill's injuries have been from collisions/contact and aren't so much age related.
Healing time is more of an issue with aging and that also has more to do with the individual than their age. Our healing rate slows down at different rates and different ages. Kamara still heals up pretty quick, so I'm not worried about his age. What I'm looking for with Hill is how long his full rehab takes. So far, he has finished rehab at or before the average minimal time. If he does that again, then no worries for me about his age.
I think the Saints are going to wait until there's a strong indication that his rehab is or isn't going well before they make a decision about him. I think if they are confident in his recovery they'll keep him and take it year by year with him. There's several things I can see them doing if his recovery is going less than great.