Goq, this is a typical function of older people. They may have very little going on in theior own lives, or little control, or as I said, it is just a function of many people being elderly. Thank them politely for their input, and then do what you want. You cannot reason with some elderly people, just as you cannot reason with some children.
BTW, does it ever occur to any of us who gets unsolicited advice, that maybe the advice is actually GOOD and we can benefit from the experience of an elderly person? Yes, you are a grownup, but a parent or “parent surrogate” never outgrows the need to worry about the young’ns. And if they chastise you for not going to your relatives for meals – maybe you could re-think whether or not your relatives are missing your company and maybe a visit or two might be in order. Of course it is not their business, unless they are your relatives.
Never be short with them about their interference. At worst, just ignore their unwanted advice. Personally, I have found such advice to be invaluable at times. I am sure it can feel intrusive and none of their business, but it’s what old people do. If it gets to be beyond the bounds of all good taste, or they are abrasive with you, then politely thank them and change the subject or walk out of the room.