True "Adult Baby" pretty much always includes diapers, in my experience / observation, because they are part and parcel of being a baby.
But not all "littles" are "adult babies" - Age Play can include regression to whatever age suits your needs / desires. I've known "littles" whose age play age is in the potty training transition phase - so they don't wear diapers, but they do wear training pants and sometimes pull-ups.
Then there are "older littles" like me. My age play age is typically 6. No diapers here. I wear "big girl" knickers. (But they don't always stay dry
)
Then there are those who age regress to pre-teens or teenagers ... none of them wear diapers.
So, to answer your primary question ... yes, "adult babies" pretty much always wear diapers ... but there is a whole world of age play to be explored other than "adult baby". The "adult baby" stereotype is how the vanilla community perceives us all ... but like most stereotypes, it is an oversimplification which does not do justice to the full range and complexity inherent in our particular kink.