It has to be renewed every 5 years for kids up to the age of 21 with passport renewal and once after age 50 with passport renewal. The reason for this is that the appearance of the person changes, so the renewal is to update the picture.
But I have heard the immigration officers in Delhi are very lenient on that if you keep the old passport with you.
I appreciate your info, and I am not arguing with your reason as the reason you gave is genuine and makes sense. But lets keep the reasons aside and talk about whats mandatory. Because reasons are subjective. There is also a counter-argument that if you issue a lifetime visa, it has to be lifetime (no need to renew).
There is another forum called immihelp. On that there is a thread where there are multiple posts which mention different cases where old passport OCI was accepted - kids, 50+, etc. Some of them are travel agents who hear about real experiences from people. I emailed CGI Vancouver saying that I dont have enough time to renew OCI for my kid, can I enter on old OCI. They replied that I can.
Is it recommended? Yes
Is it mandatory as per authentic information on websites? Yes
Is it mandatory as per ground reality at airports? No
Its unfortunate that authentic sources are misleading. One can take their own decisions on whether to renew or not, but I am posting this info so that people can make informed decision.