How to extend your 18-year-old son's UAE visa

Dubai - To avail of the extension for a dependent, the parent must submit graduation certificates to the GDRFA.

My son is in Grade 11 in Sharjah. He will turn 18 on July 25, 2021. His current Dubai-issued visa is valid till March 7, 2020. When I renew his visa, will it be valid till March 7, 2022 or until he turns 18 on July 25, 2021?
 
As your son will still be a minor when the current residence visa expires on March 7, 2020, you - as a sponsor - may apply for the renewal of his dependent residence visa in the UAE. Such renewal of the dependant visa may be valid till its expiry on March 7, 2022.
Further, the Federal Authority for Identity and Citizenship announced on June 26 that parents may apply for the one-year residency extension of their sons, who have completed their secondary or university education and are above 18 years of age.
Such residence visa is renewable for another year from the day the son graduated or on attaining 18 years of age.
To avail of this visa for your son, you need to submit your son's duly attested and legalised graduation certificates from a university, whether inside the country or abroad, to the General Directorate of Residency and Foreigners Affairs (GDRFA) and pay the respective visa cost. For further clarification, you may contact GDRFA. 
Know the law:

To avail of the residency extension for a dependent, the parent must submit graduation certificates to the GDRFA and pay the respective visa cost.