This doesn't seem to be addressed explicitly in the books. However, we do see two occasions where wizards raised amongst Muggles are informed of their wizardness, and in neither case are they told that they are not allowed do magic before going to Hogwarts. Hagrid did not tell this to Harry, nor did Dumbledore tell it to Tom Riddle.
Now it is possible that they didn't mention it yet it is still a rule, or it is possible that they mentioned it "off-screen" (although in both cases it does seem as if we are seeing the entirety of the interactions). Moreover, Dumbledore at one point explicitly warned Tom about certain rules and even then he did not mention a rule against using magic:
"At Hogwarts," Dumbledore went on, "we teach you not only to use magic, but to control it. You have — inadvertently, I am sure — been using your powers in a way that is neither taught nor tolerated at our school. You are not the first, nor will you be the last, to allow your magic to run away with you. But you should know that Hogwarts can expel students, and the Ministry of Magic — yes, there is a Ministry — will punish lawbreakers still more severely. All new wizards must accept that, in entering our world, they abide by our laws."
Additionally, in Harry's first Potions class we find the following:
"Thought you wouldn't open a book before coming, eh, Potter?"
Harry forced himself to keep looking straight into those cold eyes. He had looked through his books at the Dursleys', but did Snape expect him to remember everything in One Thousand Magical Herbs and Fungi?
This indicates that it was expected for students to do some form of magical learning on their own, even prior to ever coming to Hogwarts. This is not dispositive, though, as it is possible they were only expected to do things that would not involve the actual performance of any magic.
Furthermore, the Hogwarts letter that Harry received also made no mention of such a rule. And finally, Hermione seems to have used magic before school, and she is hardly the rulebreaking type:
"Are you sure that's a real spell?" said the girl. "Well, it's not very good, is it? I've tried a few simple spells just for practice and it's all worked for me.
In sum, there are a few indications that it might be allowed, but I don't think there is any explicit proof.