Can I say that "a little boy" refers to the age whereas "a small boy" refers to the size? Or both can be interchangeable also in meaning?
He is a little boy.
Also I would like to know about:
He is a little friend.
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Yes. A little boy means a young boy, and a small boy refers to his size.
See here (Cambridge dictionary):
little (adjective) young:
When you were little, you and your brother were always fighting.
My little brother/sister (= younger brother or sister) is seven years old.
He stayed home from work today because his little boy/girl (=young son or daughter) is sick.
No, it's not like the way you understand. Both talk about the size in this context.
However, I guess 'small' is more common in BrE (the smallest watch) whereas 'little' is what you find more in AmE (the littlest watch). But as I said, this is just a guess.
Another thing is little some times depict emotions as in 'a poor little girl'
I checked COCA where 'little boy' outnumbers 'small boy' in usage.
Out of the context of a boy, they aren't interchangeable always. You may have little experience in driving a car, but you cannot have small!