I’m 61 and incredibly flexible for a guy (and more than most women) with a very strong core to hold aero position or a sustained climb for an extended period without discomfort. Here’s how I have achieved it over the last 3 years:
Robust strength training program. It’s not part of a cyclist’s DNA to strength train year round, but like athletes in non-endurance sports know, a strong core is key to all movement. Ignore the cycling wisdom of it being just a necessary evil in base training thing and develop a full body program (with a strength coach or trainer if you are not very knowledge about routines and proper form).
Stretching and rolling after EVERY cycling workout! When your muscles are warm post a hard workout is the greatest opportunity to improve flexibility. If you build it into your routine - workout, quick recovery drink, stretch and roll, it will become second nature. A great place to start is Yoga for Cyclists.
Some key stretches to include to address your aero needs:
1 hip stretch (on one knee, upright, stretch up, back and to the opposite side of the kneeled knee
2a, 2b Child’s pose to Cobra and back again . . . slowly (Yoga moves - look them up if you are not familiar with them)
3a, 3b. Cat/Cow (also Yoga moves)
Note: Perhaps you will need to start slowly given your condition. FWIW: I have mild arthritis in my neck and lower back. All related challenges have been overcome through strength and flexibility training.