What age is too old to live alone?

It’s not about age, it’s about ability. If a person cannot do the basic care— getting in and out of bed, taking care of hygiene needs(toilet and washing), dressing and undressing, and feeding themselves they should have help either full time or part time. If forgetfulness is an issue and medication is not taken regularly or the stove is not shut off, etc., then this person would graduate to needing full time help. Many very old people can still care for themselves and some young-old cannot. It’s not an age thing, it’s a physical and mental ability thing.