Translation:I am ten years old.
No, it's not. Japanese language usually ommits the subject. Some even say there are no subject at all in Japanese¹. So "Jū sai desu" could "I am ten years old", but it could also be "She/he/they/you/we is/are ten years old". It depends on the context. If someone asked "How old is your son/daughter?", you would answer "Jū sai desu". The same goes for "How old are they?", etc.