When did the middle age start?

Probably the best answer is that the Middle Ages started in the 5th century. To pick a specific year, it might be best to choose 476 AD.

There are very different answers to this question.

Some historians have regarded the Middle Ages as a period after the Dark Ages. These people would give dates of 1000 AD or 1066 AD as the beginning of the Middle Ages.

I think most historians would make the Middle Ages a period of about 1000 years from the 5th century to the 15th. They replace the term Dark Ages with Early Middle Ages. Some of these people simply say the Middle Ages started in the 5th century with a process that lasted many decades and cannot be dated. Others provide specific dates of specific events. I have seen dates ranging from the late 4th century to the early 6th century, but 476 is by far the one I see most often.