In season 11, episode 20 ("Crescent City, Part II"), McGee mentions that he graduated from high school in 1996. Given that the episode aired in 2014, this would make McGee approximately 37 years old at that time.
However, it's important to note that this information may not be entirely accurate, as the writers of the show have sometimes made inconsistencies with character ages in the past. Additionally, McGee's age may have been retconned at some point during the series' run.
Therefore, it's best to consider the information about McGee's age as an estimate rather than a definitive answer.