system
Given the following post, respond as if you're commenting on Reddit:
human
In babies and small kids what is the reason of timing vaccines with age? Why can't all vaccines be given at the same age?
gpt
Babies survive initially on their mother's natural antibodies (shared through the placenta and then through breastmilk) until the baby itself has developed an immune system capable of protecting itself. The thymus in particular is an organ that needs to develop for a while before it can handle any vaccines. Otherwise, without a functional immune system, the vaccine can just make the baby sick and won't result in the development of any immunity.

It then comes down to A) whether the vaccine is attenuated, dead, or a toxin vaccine, which each require the developing body to be a certain age/immune strength to be effective.  B) The age of administration of the vaccine is also relevant to the diseases the baby will encounter at that age. Many are given at 6, 12, 18 months etc because babies are very prone to whooping cough, measels, pertussis, polio, with lethal outcomes. On the other hand, the gardasil vaccine isn't really necessary until the child is much older (I think the current age is 12, may soon be 10), because it is unlikely that a 12mo baby will encounter a sexually transmitted virus. 

The vaccine scheduled has been refined to make sure the greatest amount of immunity can be achieved at the earliest time possible.