At mealtimes in the Middle Ages, all drank small beer, regardless of age, particularly while eating a meal at the table. Table beer was around this time typically less than 1% ABV.
Beer during the Middle Ages was naturally produced in a wide range of alcohol concentrations, generally classed as strong beers of 8-14% ABV; medium beers of about 4-8% ABV, and weak beers of 1-3% ABV.
It had about 10% alcohol content, so it was quite strong. And we know what it was made of because the Egyptians were very methodical at writing down how they made their beers. In the 1980s people were able to recreate the beer of Tutankhamun based on hieroglyph information.
There is a story repeated so often that it has become a truth — that medieval folk drank weak beer to avoid the perils of drinking water — but it's a myth. The folk in olden times who could afford it would pay for good clean water.