Average age of a successful game developer?

Of course, it depends on what kind of developer you join and your own skills and experience. Indie developers in particular will vary drastically in terms of structure and personnel.

Generally though, in positions of responsibility and especially critical roles like project management and design lead, you want guys with a proven track record and experience who have been in pressured and challenging situations before. Of course, if someone has been in the industry from a very young age, or has shown exceptional ability they can take on responsibility at an earlier age. On the whole though, you’re not going to join Rock star at 21 as a junior programmer and be the technical lead on GTA VI by the age of 25 unless you’re a prodigy.