How old did you had to be to queen in Medieval Times?

Answer 1

There was no minimum age. But a queen or king who was still very young would have an appointed guardian or ruling counsel to rule until he/she had come of age, which was usually around 18 years.

If you mean a Queen-consort, a princess married to a King: they could be betrothed at a very young age, but actual marriage would seldom take place before the princess was at least twelve years old. Consummation of the marriage even then usually was postponed until the princess had reached puberty, which at the time did usually not occur before age fourteen.

Answer 2

There was no actual age limit. Girls who were too young to rule had a person like a mother, grandmother, male relative who made decisions for them until they were older.