When historians use the term "Bronze Age" they do not mean it was the only metal humans were using, they mean it was the latest and most common. Iron ore is very, very common, but until the iron age we did not know how to process it, so iron was very rare, only usable when found in its pure form such as in meteors. So while the Egyptians knew it existed, they simply did not have access to enough pure iron to use it for anything other than ceremonial uses. Once the technology for smelting was developed, iron quickly became the most popular metal because it was cheaper, stronger and more workable than bronze.