That's a good question, and if it doesn't exist we need one! If they were female, I'd probably say "little/sweet old lady", I know that doesn't exactly get the same meaning across but it has connotations of sweetness or innocence? I can't really think of a male equivalent though unfortunately, short of "little old man" but I don't think it has quite the same associations, though that seems really prejudice. The only other one I can think of is "old dear" as in "he's/she's such an old dear, bless him/her" but I'm not sure it conveys innocence as such. So in short, no we don't really have one that I can think of, but maybe someone else can think of something?

I cannot think of something right now, would love to hear what others think.

It doesn't refer to old people specifically but I think calling someone a "gem" could fit here. "Oh, have you not met Ruth? She is such a gem!"

I have heard people use the following to describe nice older ladies in this way. "You are a dear." "You are a darling." "You are a treasure."

Sometimes "sprightly" is used to describe an old person who is lively and cheerful, thinking that it's cute to see someone like that. To show innocence, a person can be described as "childlike" (NOT "childish", which has a different meaning) or having a childlike demeanor. Like Coral says, something like "sweet little old lady" (with one or both of "sweet" and "little"). That can also be said of a man, but less usually. note: had to edit this because it changed my ampersand to a greater than sign (!)