Are female Havs who will have their first heat at 12 to 15 months old slow to mature in other ways (for example - teeth, body size, etc.)? Are there physical or learning problems associated with late maturity?
I don't necessarily think that the age of their first heat and maturity always correlate. For instance Starla, my non-bred by girl had her first heat @ 12 months of age. She reached her adult size @ 8-9 months. I don't think she's fully mentally mature yet. She has just begun to mellow out some.
While my bred by girls got their first heat @ 15 months. At 19 + months of age they are finally filling out. I'm not sure they're quite growing as their mother and another bitch of the same lineage didn't fully reach their adult size at around 2 years.
Not sure if you're looking for a pet or a show girl to breed but its actually preferable that their first heat is later than earlier as their uterus ages with each cycle. And, more rapidly so with earlier & shorter heats.
I haven't seen any correlation. I know if a dog's reproduction is mature and they are around another in season, often this will bring the dog in season also. I've had littermates have their first season at varying ages, but some lived with another female that came in and others didn't and both matured similarly.