I can't agree that "old people don't notice speed." It really depends on the person. If the OP's mother calls to ask why her PC is slow now, she will appreciate an SSD. Probably more than she'd appreciate a faster CPU.
My mother didn't multitask and didn't do much more than browse the web and read email, but she didn't leave her computer on all the time and hated waiting for it to boot up. She may have been in her 80s, but when she wanted to use the computer, she wanted to use it now. SSDs seriously cut down on start-up time. They also can speed up app loading, though that depends some on the program and how many files it accesses at startup. They also actually help some with browsing, especially if you tend to look at the same sites a lot, since cached files are re-loaded faster.