Age usually refers to the number of years that a person has lived. The following examples show some of the ways of referring to someone's age.
1. My mother is 40 years old.
2. My mother is a 40-year-old school teacher.
(Note: not 'a 40-years-old school teacher'. Also note that '40-year-old' here is a compound adjective, which we discussed on Tuesday.)
3. She teaches six-year-olds.
(Here, 'six-year-olds' is a noun. Remember to use hyphens to make it one unit.)
4. She teaches students aged six.
5. She became a teacher at the age of 26.
6. She was 26 years old when she became a teacher.
7. She started teaching six-year-olds when she was in her 30s.
('In her 30s' means between the ages of 30 and 40. Note: not 30's. 'In her early 30s' means she is over 30 but below 35, while 'in her late 30s' means she is over 35 but below 40.)