It does happen as one ages, but if it's a significant change that happens quickly it should be checked out.

Consider this as well: it's very easy for people to be in denial when something is going on with a family member, especially a parent. I spent a year minimizing everyone else's concerns about my father (saying "that just happens when people age" or "well, that happens to me too, and I'm fine!") until it became impossible to ignore what was happening. Turns out he has Parkinson's. I still beat myself up a little for not saying something sooner, and maybe we could have gotten him into treatment a little sooner.

So yeah, I recommend OP gently bring it up with mom ("Mom, I'm just a little concerned about the things I've been noticing"). If nothing's wrong, then at least you'll know that. But with cognitive issues like dementia etc early detection means best treatment/prognosis, so I always recommend erring on the side of caution.