I've often waited til 3-4 yrs to do finals.......and only once has a rating "declined", several times hips have improved and for me.....elbows have stayed the same.
Please correct me if I am wrong but from what I have been taught, your dog will be compared to other Labradors of the SAME AGE. So if you do not get hips and elbows done till 5, 6, 7 years of age the OFA takes in consideration the age. Meaning the will not compare the hip conformation of a 4 year old lab to what it should look like at a younger age such as 2.