I'm 43 years old and ride 2-4 times a week, 1-1.5 hours a day. Figure tower high or thereabouts heelside, toeside (don't ask!!). Used to ride a Trips balsa 138 with 01 Belmonts. My knees would feel like they were burning midsession and would be sore for days after. My hip on my landing leg felt sore and ached as well. Here's what I changed: I used to ride rear foot back more than front foot forward-- like this: \--!. I changed to this \--/ and evenly balanced. Learned this test from a thread a few months back: stand and jump up as high as you can then land: how are your feet positioned? For me it was legs about shoulder width apart and feet ducked out. Greg notes this as well. When I landed feet straight forward or more on one leg there was pain in my knees and/or hip.
I also switched to the CWB Zues with the CWB Absolute 140 board. Incredibly soft landings. When I case the wake, I feel like I'm landing on sponges. Landings in the flats are no problem. Landing the trips and I felt like I was getting slammed, in comparison. Little or no hip/knee pain now.
Greg also points something else out: land knees soft. Land straight leg and things get a little whacked. My knees "unlock" on a jump if I hit the wake, or "pop" it right.
Riding more should help get your hip "in shape" that is if you don't ride a lot already.
Anyway, good luck. Just know there are options.