So make a separate class with a method that can check the age and determine what it should be if it returns true make a new block with the type and attributes?
Yes. Changing object behaviour with if-statements is often a sign of missing design to benefit from inheritance, object composition, polymorphism, etc.
And have it in the for block, in the worldmap.render method?
No, do it before. Modifying the game world is game logic. Rendering should, well, render. Clearly assign tasks and do not mix them.