A: Dear Parent:
I would not worry about a 13 month old who tries to play with 2 to 5 year olds but doesn't interact with children his own age. Children of 13 months have their hands full just watching other persons of all ages, and discovering how their own bodies work, and being sociable with the important people around them. I would not make a special issue of his interactions with youngsters his own age.
The whole idea of a narrow peer group is an idea which is only about 100 years old. Previously, children were raised and educated in groups of children of various ages--in the one-room schoolhouse, for example. The model of mixed-age experiences is now being revived, in fact, by some educators.
If your son is friendly and interactive sometimes with some kids, I would say that this sounds excellent. You will probably see his social sphere widen considerably over the next several months.