It's a fact of life that as we get older, we tend to lose range of motion and strength. We're not necessarily condemned to stiffness in our movements, but success may require increased attention to stretches, and to exercises to maintain or improve mobility.
Fundamentally, there's not a lot of mystery to getting edge angles--it's inclination plus angulation. Inclination is always going to be the largest contributor--and here I define inclination functionally as the angle between vertical and a straight line drawn through the base of support and the center of mass. I don't mean banking, which would be inclination without angulation. Inclination comes naturally, it's just what's needed to stay in balance against centripetal force. What's needed to increase edge angle further--significantly past the "critical angle"--is angulation. As has been written about extensively, there are several contributors to angulation. The two main ones are "knee angulation" and "hip angulation."
Knee angulation does exist, but the term is misleading. As Rod9301 notes, the knees don't much bend laterally. Instead, what we call knee angulation, and what can give an illusion of the knees bending that way, is a turning of the femur in the hip joint accompanied by an opposing turning of the lower leg relative to the foot, through the subtalar joint (accentuated by a bit of tib-fib rotation). The result is that a line drawn from the hip joint to the toes runs to one side of the knee, offset. However, too much dependence on knee angulation can place excessive stress on the knees. Others have noted that it can also actually block upper-lower body separation (or counteraction). Knee angulation is nevertheless important for fine adjustments to edge angle, and for preventing washout of the outside ski. Others have noted that initiation of and fine control of knee angulation is promoted by the skier being in touch with his subtalar joints, and triggering movements with foot tipping.
Hip angulation has the potential of contributing a much larger component to total angulation. Here is where it's critical to have range of motion in the hip joints, and good muscular control over movements in those joints in every direction. Have a look at
https://skierlab.com/hip-flexibility-in-skiing/ . This presents some introductory ideas on stretching to promote freedom in the various planes of motion. Then have another look at the Mikaela video posted above. You might see her engaging in active hip leveling. To create hip angulation, shortening the inside leg is essential, but it's important not to allow that to lead only to banking, or to angulation created only through a bend in the spine. Active hip leveling will require that the skier have access to sufficient degrees of abduction and adduction through the hip joints. It will also require a component of muscular strength to move into and sustain abduction and adduction. These are movements we don't engage in so much in our everyday sedentary lives. Finally, good inside foot management is needed to keep the inside ski from wandering away, which will lead to the skier using it as an outrigger and leaning to the inside of the turn, thus wrecking the benefits of all that work aimed at creating angulation. This requires a certain amount of muscular effort to create adduction of the inside leg, keeping it close to the outside leg.
And yes, get skinny skis, they place less stress on the knees and just in general require less effort to tip them up on edge.
I think these are a few points to work on. Undoubtedly there are many others.