So here is some stuff:
I looked at some knuckleballers of the past to see where Dickey might be headed. I focused on 38 and over(to see how they did compared to their norms) and things such as velocity and workhorseness. I also tried to focus on times closer to the present. Remember this all depends on the information I have at hand and at times alot of it is really poor.
So guys I'll be looking at: Wakefield, Hough, Condiotti
Wakefield(1992-2012):
Age 39: 7-11 140 IP 4.63 ERA. 8.7 H/9. 1.2 HR/9. 3.3 BB/9. 5.8 K/9.
Age 40: 17-12 189 IP 4.76 ERA. 9.1 H/9. 1 HR/9. 3 BB/9/ 5.2 K/9.
Age 41: 10-11 181 IP 4.13 ERA. 7.7 H/9. 1.2 HR/9. 3 BB/9. 5.8 K/9.
Career: 12-11 201 IP 4.41 ERA. 8.8 H/9. 1.2 HR/9. 3.4 BB/9. 6 K/9.
Hough(1970-1994):
Age 39: 18-13 285 IP(UMMMMMM) 3.79 ERA. 7.5 H/9. 1.1 HR/9. 3.9 BB/9. 7 K/9.
Age 40: 15-16 252 IP(ERM) 3.32 ERA. 7.2 H/9. 0.7 HR/9. 4.5 BB/9. 6.2 K/9.
Age 41: 10-13 185 IP 4.35 ERA. 8.3 H/9. 1.4 HR/9. 4.7 BB/9. 4.6 K/9.
Career: 11-11 199 IP 3.75 ERA. 7.8 H/9. 0.9 HR/9. 3.9 BB/9. 5.6 K/9.
Candiotti(1982-1999):
Age 39: 10-7 135 IP 3.60 ERA. 8.5 H/9 1.4 HR/9 2.7 BB/9 5.9 K/9.
Age 40: 11-16 201 IP 4.84 ERA. 9.9 H/9 1.3 HR/9 2.8 BB/9 4.4 K/9.
Age 41: Doesn't matter. Small sample size.
Career: 12-13 215 IP 3.73 ERA. 8.8 H/9 0.8 HR/9 2.9 BB/9 5.7 K/9.
Essentially the drop off for these knuckle ballers in their latter ages isn't as extreme as I thought. Wakefield and Hough struck out slightly less, while Candiotti has a bit of a drop off at 40. Walk numbers stayed similar or rose slightly. As expected their workhorseness (It's a word) decreased with age. HR/9 rates for all three rose a bit. H/9 varied season to season between the three.
Again this is an incredibly superficial look into 3 knuckleballers and how they progressed at 39, 40 and 41. Dickey's last 3 seasons were better than them (though I dunno if he can give us 285 IP

). So it's somewhat encouraging, but remember this is quite a superficial look.
A fun tool in baseball-reference is to use certain factors to predict how a pitcher will do in a different park.
Last year, Dickey would've performed like this in Toronto according to BR:
15-9 3.29 ERA 219 IP 204 H 80 ER 26 HR 57 BB 215 K 1.192 WHIP.
Not too shabby.