Tom Gates is 11 years old in the first book of the series, Tom Gates: Totally Fabulous (and Slightly Disasterous) Diary. He ages throughout the series, so by the end of Tom Gates: Top Secret (and Slightly Smelly) Diary, he is 13 years old.
Tom's age is never explicitly stated in the books, but it is implied through his experiences and the events that occur. For example, in the first book, Tom is starting middle school, which is typically for children aged 11-13.
The author, Liz Pichon, has also confirmed Tom's age in interviews and on social media.
