Your first two are wrong. The third is ok but rather literary When I was 10 When I was about/around 10 When I was a child of (about) 10 You can add ‘years old’ to the above but it usually wouldn’t be necessary.

The easiest was is to use one of these structures: When I was a kid/a teen: When I was a kid, I started learning Spanish. When I was a teen, I continued learning Spanish. When I was (age): When I was 21, I did an Erasmus in Barcelona. When I was 22, I started teaching Spanish.

If you want to talk about the period from when you are 13–19, you can say, "my teens" or "my teenage years". Children from about 9–12 are often called "preteens" or informally, "tweens". Some examples: "I was a bad student during my teens." "As a teenager, my son lost his sunny personality and started brooding." "When I was 12, the kind of music I listened to started to change. "I have no patience with these tweens and all their drama." "In my teenage years, I was stubborn, and nobody could tell me anything."

There must literally be hundreds. "When I was ten," of course. "In my boyhood..." "In my childhood..." "In my tweens..." "When I was a kid..." "In my middle-school years..." "When I was a boy..." "In my pre-teen years..." "In my pre-adolescent years..."

Old song: "School days, school days, Dear old golden rule days... ...You were my queen in calico, I was your bashful barefoot beau, And I wrote on your slate, 'I love you so,' When we were a couple of kids." The context makes it clear here that "kids" might well be age ten, or eleven, or twelve.