Can my 90-year-old mother, suffering from dementia, be used as a witness in a court of law?

Boy, if I were on trial for something and the only witness was someone with dementia, I'd start planning for jail. My mother-in-law has what the doctor is now calling the beginning of Alzheimer's. Her memory stinks, so if my life depended on her testimony in court, I'd be doomed. However, if she was up there testifying to what happened when she was a kid, then all would be good. So I guess it would depend on how bad the dementia is, and if the lawyer opposing her was any good.