age

People in the 14-17 group of age were most affected by the changes.

I know, this seems to make sense, and your meaning is clear, but if you want a high score for vocabulary you need to use better collocation inside your noun phrases:

• People in the 14-17 age group were most affected by the changes.

The following are all possible. Unfortunately you just have to memorise them. Try to use them as soon as possible in your writing practice and soon they will become automatic.

• a person who is 49 years of age
• a person who is 49 years old
• a person who is 49
• a 49 year-old person
• a 49 year-old

More often in IELTS, ‘person’ is plural (= ‘people’), and so these noun phrases are also possible:

• 49 year-olds
• 49 year-old people
• people in the 50-60 age group
• people aged between 50 and 60
• people aged 50-60
• people (who are) 50-60 years old

Of course we could be more specific about ‘person’ or ‘people’:

• A 49 year-old English teacher
• English teachers in the 49-59 age group