Just as an example Sinovac is not even used in old people in China as far as I know.

There’s a similar question with AstraZeneca right now, German vaccination committee would like to see more Phase 3 data on old people before moving into Phase 4.

On the other hand, they might just wring out the best response possible at a slightly greater risk. Here I'm assuming your parents shouldn't try a PRC vaccine, so it's virus vector Russian or mMRA Pfizer, both essentially accomplishing the same thing, both with high overall efficacy.

Or look at it this way: if they don't get a good result from either of these two vaccines, they're not likely to fair well from a real infection. Maybe. We really don't known why even at their age most people survive.

The other factor is how much you trust quality control. Pfizer > Russia >> PRC in my book, but I have absolutely no read on Russian quality. But also note Pfizer/BioNTech is useless if not properly handled to its strict temp and physical requirements.