The main car park is a bit of a hike up the hill about a quarter mile from the beach. There is a small private car park nearer the beach, but it only has limited space. Closer to the beach there are a few yellow lines your can park on if you have a disabled permit. Your biggest challenge is walking the final stretch which is over an area of fairly flat rocks. Have a look at this picture and you will get an idea:
My mother is also in her early seventies, with two replacement knee joints and she managed fine.
Your best bet is driving right down to the bottom, dropping your mother off, then driving back up to the main car par and walking down yourself.
Trebarwith Strand has one of the nicest beaches in Cornwall so it's well worth the effort.
Thanks Kevin, that's really helpful and much appreciated. Good idea to drop mum off first and that's what I'll do. Thanks also for the picture - it's so much easier to get an idea of what it's like. I can't wait. Thanks again, Gaynor