I'd see OP's question as open to interpretation. What level of understanding, and understand which aspects of the book? I'm unsure when I first read it - age 11 maybe? - but I don't think at the time I knew much about say Stalin. I do think I would have been able to work out some of the general themes about human nature though and still gain quite a bit from them. In fact, I'd say that an intelligent person from say a non-European country with no knowledge of the Soviet Union could still read Animal Farm outside of historical context, and understand a whole load about the themes, interactions between the sets of characters and more.
To my eyes it would actually be a pretty crappy book (as literature at least) if to read it you needed as a prerequisite a whole load of knowledge about history and political theory and what have you. If a book is an allegory, then there are a minimum of two levels on which it can be enjoyed or learned from (and the hidden meaning is often one that can be worked out for yourself). And personally I don't reckon that one particular understanding or another should be said to be the best or most important one. Throughout my childhood and teenage years I read a whole ton of books blithely missing out on a whole range of some of their meanings. But I also gained a hell of a lot from them too, and I don't think that my not understanding or noticing some of these meanings signifies that I oughtn't to have read them at all. A great deal of the books I went on to re-read and discover more meanings, and having read them in a sense through different eyes possibly that meant that overall I learned even more.
So as well as being unsure that there is one meaning behind Animal Farm to be discovered, even if there was I don't think that's a reason for a person to read or not read it at a particular age or not. Sure you can read a book and miss out on some fairly central stuff, and maybe feel a bit dumb later on because of it, but I'm all for encouraging kids to read whatever books they like at whatever age they like, just say nudging them in potentially interesting directions every once in a while instead of dictating.