あの人は10代後半(前半)に見えるって英語でなんて言うの？

If you use the first example you are comparing this persons looks to that of someone who is 18, or 19 years old. "He looks like he is in his late teens." In the second example you are saying this person looks about 20, 21, 22 years old. "He looks like he could be in his early twenties."

When speaking about someone's age, if you say 'probably' it means that you are guessing the age. "Henry is probably about 35 years old."

All of these are different ways to describe people who are between the ages of 13 and 19. For example: Zach looks like he is in his teenage years. Zach is in his teens. Zach is a teenager. All of these are acceptable ways to talk about people in that age range. I hope this helps :)

"He looks like he is in his late teens."This is the more common phrase used to express the age that a person looks like in English. "He looks like he is (almost/close to) twenty." Here you are saying that in the person looks as though they are nearly 20 years old.

a teenager is someone who is between 13-19 and also referred to as being a 'teen' so by saying they look like they are in their late teens suggests they are about 19-20 years old we can also say in their 'early twenties'

"Early 10s" Early (number) is used when describing someone's age, early describes ages from 0-4 of the age, ex. 10,11,12,13,14 years old, Mid (number), usually if they look '5' for example '15', Late (number) if they are 6-10, ex. 16,17,18,19,20 years old.

When guessing someone's age we can look at their appearance to help us gage their age. The word 'probably' tells us that this is an assumption. We can use the phrase 'late teens' to point out that the person in question has aged well into their teens, past age 17-19.