If the Minor has source of earning, it's good idea to have a PAN Number. It's not mandatory but pretty much required for any individual with a source of earning.
When Minor reaches the age of 18, you can apply submit a 'Change Request Form'. While the PAN number remain same, the PAN Card for adult will have a photo on it.
To get the correction done in PAN Card one needs to fill up a 'PAN Change Request Form' from the official website of NSDL or UTI Technology Services Limited, who are authorized for it.
For any kind of change in PAN Card one needs to provide the relevant documents listed in the change request application form.
Link to download the PAN Card Change Request Form:
NSDL PAN Service