6-8 months for most smaller breeds, bigger breeds may be older.
At Norwegian Rabbit Shows the rabbits can be put in adult class from the day they turn 6 months. (Previous being Young animal from 4-6 months)
Rabbits can be bred safely from they are 6-8 monhts so therefore I consider them adults at that point.
I consider them "teenagers" from 3-6 months, since this is the time they go trhough puberty.