Bobby and Cindy

The actors portraying Mike Loookinland (Bobby) and Susan Oleson (Cindy), appeared too young for the scenes of later seasons. They both still appeared to be the same age as they were in the pilot. In real life, both of these actors were adolescents by season 5. They even adjusted the hairstyles of the 4 older kids to change with the seasons.

It would have been more realistic to bring in teenage actors for later years, since there is a huge appearance change between pre pubescent years and teenage years.