For the last 5 months I worked on a project in a team where me and other 3 developers had to build a mobile application starting from scratch.
I am the the youngest developer in this team, being between 10 to 15 years younger than other members.
During development I had the possibility to grow and "shine" enough to let my manager (who is part of the team) decide that in one month, when the application will be delivered to store and to client, I'll become the PM of this project and directly responsible of managing the team during bug fixing and evolution of the app, due to my current manager's resignation.
Despite me being very happy for that (I have 25 years and just 2 years of experience in this +400 developers company, making me very proud of my work) I recently heard from other 2 colleagues at the coffee machine, while I was at the near bathroom, that one of them is openly angry at me and he does not want to take a single order or incipit from my person.
That person used to be one of my "friends" during work (we went every day out for lunch and we used to talk about our private lives during coffee, not a close friend but something similar) and I know that the problem here is just because of my age compared to his, we both know that my skill is better .
How can I handle this situation? Note that I never had this position before, and I don't want to argue with him, not in public anyway.