It is interesting epidemiology. There are studies, but it is understudied. I think patients with suspected cause like antibiotics, which could occur at any age, need to be excluded to get a clearer picture. It is also harder to compare studies because some focus on the degree of treatment needed while others focus on the chances of remission (no matter what treatment).
In work on a small sample from 1985, late-onset was more likely UP, less likely to respond to enemas, took longer to get initial remission, and more likely to have shorter remission.
/www.ncbi.nlm.nih.gov/pubmed/4086743By 2010 another study found that Late-onset patients have better responses to therapy 1 year after diagnosis.
/www.ncbi.nlm.nih.gov/pubmed/20363368 - but medications and treatment protocols had also changed over time.
Life factors, like prior smoking, seems more related to late-onset, while family history more related to early onset. But, both environmental triggers and family history (DNA) are important for both groups. Just some difference in relative importance on average.
There are gender difference too - especially in early onset.
I would suspect there are huge differences by health care coverage status, because uninsured are often undertreated.