It is possible that the leg shaking comes as things surface during meditation and will go of its own accord eventually if you just observe it and don't react with wanting it to go away.
It is also possible that it is simply that your legs are no longer used to sitting in this position for a long period now you are older.
Have you a cushion under your bottom to raise it slightly? This helps to keep the back naturally straight by tipping the body into a better position so the back doesn't round.
Do your legs or back ache after the sitting or are they perfectly fine once you stop meditating?
If they only react when you are meditating - it is most likely due to the meditation.
If there is some problem still when you stop it is more likely due to your position and you could experiment with different positions ie
legs not crossed,
on a chair - but not leaning against the back of it,
using a meditation stool so your bottom is on the stool and your legs tucked underneath or to the side of it.
Using your preferred cross-legged position and standing to meditate when your legs shake - this has the advantage that you will get stronger and notice that you need to stand after a longer and longer time. I would still use a cushion under your derriere though.
Have fun experimenting
peace and joy
