Answer:
Thomas is 10 years old.
Explanation:
The word "decade" is derived from the Greek root word "deca" meaning 'a group of ten'. So, we can easily assume that any word with that root word will be related to the number 10.
Thomas wants to celebrate his becoming a decade older. This means that Thomas is ten years old. The word "decade", means a number of ten years, or a period of ten years.
Thus, Thomas is 10 years old.
