Junior and Senior are show/breeding terms. A junior is a rabbit under 6 months. A Senior is over 6 or 8 months depending the on the breed. An Intermediate is a large breed between 6 and 8 months.
Kits and babies could be the same, a very young rabbit that is still in the nest box and maybe with the mother.
Teen and adult could be more about the behaviour and life stage than age. A teen would be a rabbit that is becoming hormonal and acts out becasue of it like a human teenager would be. An Adult would be a rabbit that is more settled into the personality and you would know what you are getting. Any rabbit over 1 year old would be an adult.
If you want it more in human terms, a kit would be an infant (0-2), babies would be like a toddler (2-4 years old), juniors would be a school aged child (6-10), a teen would be a teenager (12-16), a senior could be a high school or college student and an adult would be a more mature person (25+).