What does age have to do with it? If Dre produces a great song with Kendrick, what would it matter if he were 55 or 75? To be honest, a lot of people dismissed the production on Compton and DJ Dahi was all over it, but are salivating over his contributions to Kendrick's new album. Not one note has been heard. Same goes for the 9th Wonder production and if we're honest about it, what was the last mind blowing dope song he produced?
I'll argue both sides of the coin. Because I'm not that interested in Dr. Dre at this point. I think there are active producers doing way more interesting things right now, and making better music. But you're exactly right. Age has nothing to do with being a producer. If anything, being older can help, especially if you're evolving and expanding your skills, and plus you're deeper with the skills you've already got.
Regarding 9th Wonder, I think he's got a lot of dope material with artists on his record label, Jamla, that a lot of people just haven't heard or may have missed.
He had some heat on Anderson .Paak's last project, and he produced the majority of Rapsody's EP that dropped last November, Crown. He had a slept on album with Murs and Talib Kweli each, in 2015. Random assorted joints here and there that are solid.
9th IMO isn't the dude you go to for something that you might use Dre for. But I think there's a lane that 9th fits into nicely.
hurry up 
