Hiromi, you can take it as a compliment that people have difficulty guessing your age by looking at your photo   :-)

I suppose "young" is a relative term.  People in their 20's can easily describe themselves as "young".  People in their 30s may still seem young to people who are 50+ years old...and yet they might seem "older and more experienced" to people who are still in their 20s.   Someone who is 70+ years old may think that anyone under 50 is still "young."

I saw one iTalki user introduce herself like this in a notebook exercise:  Hi, I´m Ilka. I´m quite old. Sometimes, but not always. Once in a while I´m 27. Sometimes even 81. Or anything in between. ;)

Ilka described how the way we feel about ourselves can change from day to day, moment to moment, situation to situation.  Sometimes we feel young and sometimes we feel old -- regardless of whatever our actual age is.

I think people often like to know whether they are talking to an age peer (someone close to their age) or whether they are talking to someone who is much younger or much older.  This helps them think about what topics might be interesting to the other person.  Also, people sometimes talk more or less formally to someone, depending on the other person's age.

Before I added my photo to my iTalki account, I used to give people a little clue about my age by telling them that I had children in college.   That way, without my telling them my exact age, they could have a general sense of the stage of life that I am at.   This could help them decide how they would want to talk to me in our first online conversations.   You don't have to tell people your exact age, but giving them a "clue" might help them feel more confident about how to talk to you.