If I say "bless his cotton socks" about anyone over the age to start school I am being ironic or sarcastic, and I never the use the term "Oh bless!" about anyone at all whatever their age, because it always sounds patronising to me, implying an old twit who has no idea that he or she is making an idiot of themselves. I would certainly never use it about an old person - even if they were senile and into their second childhood.
If I heard someone use it about me I would ask who sneezed! That is if I could restrain myself from punching them in the nose and then saying "Oh I'm sorry, I thought you were the chap in a white coat from the nursing home, who wants me to take the little green and white pills when I'd rather get on with my memoirs of my fifty years as a nuclear scientist." (or whatever genuine career you would like to fill in here)