What is the minimum age to use MOIA?

You may book a journey for a minor as a guardian or with the consent of a legal guardian. You must ensure that minors are supervised to the required extent, if necessary. The MOIA Provider shall not assume any duties that involve supervising minors.

C.2.4.
You must also take responsibility for ensuring that minors receive adequate protection with a child seat as required by law. You can book a child seat for a minor, if a child seat as required by law is available in the service area. You will find an overview over the child seats available in the Help-Center.

C.2.5.
The transport of a minor under the age of six years is only permitted if accompanied by an appropriate supervisor.

C.2.6.
The transport of a minor aged between 6 and 14 years is only permitted without the accompaniment of a supervisor if the minor demonstrates a sufficient level of maturity with respect to his or her stage of development and a legal guardian has provided written consent. This written consent must be presented to the driver on request.

C.2.7.
It is permitted to transport a minor aged between 14 and 18 years if the minor demonstrates a sufficient level of maturity with respect to his or her stage of development and there is evidence of prior consent from the legal guardian. 

You can find per-written form of the letter of consent as mentioned in  C.2.6 attached to this article.