1. in my childhood (it's vague, but probably 3 - 12 years old) 2. during my childhood (it's vague, but probably 3 - 12 years old) 3. when I was a child (it's vague, but probably 3-12 years old) 4. during early childhood (this phrase isn't very natural, but maybe 3-6 years old) 5. during mid-childhood (this phrase isn't very natural, but maybe 7-9 years old) 6. during late childhood (this phrase isn't very natural, but maybe 10-12 years old) 7. when I was very young (maybe 0-5 years old) 8. in my infancy (0-2 years old) 9. when I was a kid (it's vague, but probably 3-12 years old) 10. during elementary school years (generally 6 - 11 or 12 years old) 11. during adolescence (maybe 11-17 years old) 12. during my teenage years / when I was a teenager (this is pretty precise because the numbers end with "teen": 13-19 years old)

You would say "when I was a child" to let the youth know that the time you are talking about is during your childhood. You would say "when I was a teenager" to let the youth know that the time you are talking about is during your adolescence, since "adolescence" is a big word for much younger people. These all appear correct except for #10. In #10, you should specify that they are "your elementary school years."

Jack and Angela, thank you very much. Finally I could understand the nuance for each. Have a wonderful week.

We'd be more willing to help you if you give your ideas as well.