As with passports, most nations prefer to have a picture that reasonably resembles the person using it to travel into the country. This is why there are usually age-5 and age-10 requirements for new passports for many countries like the U.S. and the UK. So it is not that the OCI will become invalid. It's that if a 20-something shows up at India border control and displays an OCI with a picture that is obviously that of a 2-month old baby, then the border control officer  can refuse entry based on the lack of an ability to verify the holder of the OCI. That person ***has OCI***, but they need to update it with a more current photograph.