Assuming that "Type 1.5" is T1 diagnosed past childhood:
I think "Type 1.5" is mostly a social term, not a medical term. It exists, because the mindset (incorrect since the beginning of time but still a prevalent mindset) is still that "Juvenile Diabetes" which then became "T1" is only for kids, so anyone else with the same disease who happens to be older needs a different term.
I think there are very real social implications that mean that T1.5's do form a cohesive group with experiences not exactly like T1's diagnosed as kids.
You'd be surprised how much age-of-patient-at-diagnosis has to do with treatment options and lots of other things.
You'd be surprised how much age-of-patient-currently has to do with the term used to describe them. I have been told many times over the decades, how I don't have "Juvenile diabetes" anymore because at that point I was over 18. (They told that to me in the 1980's when I was in college!). That my juvenile diabetes had turned into adult diabetes when I turned 18. I have been told many times as well that I'm not "Type 1" because kids have that and I'm obviously an over-the-hill grey haried adult. (Told to me as recently as this January at the opthamologist.) It's these preconceptions that make 1.5 a somewhat useful term for the recently diagnosed, because otherwise they do not fit into stereotypes.
How rude for the disease not to recognize the preconceived stereotypes of so many!
Even the JDRF has come around and finally admitted that most T1's are not kids or are not kids anymore, and they now actively support T1's who are adults, even though there's still a little bit of an asterisk on the designation to make it clear that they are "adults with T1" and not just "T1" (presumably meaning kids).