Retrievers are pretty simple to housetrain. They want to please their people and live in a clean environment. Retrievers often resist crate training and can even injure themselves in trying to get out and be with their people.
Late Maturers take a longer time to mature mentally and physically. So it takes a long time for them to understand the difference between where they should potty and where not. They are also late at developing a control over their bladder.
If your dog is a late maturer, you may have more than 18 months of watching it closely and sometimes cleaning up an accident. Most likely your dog will be 3 years old before you can fully trust him to not have an accident.
Some of the Late maturers include: Irish Wolfhounds, all Mastiffs, Great Danes, Chinese Shar-Peis.