息子：(the) oldest/eldest child

my boy

little boy

My son turned 18 years old this month.

will turn...   ...才になる

turned ...　...才になった

My son is in his early/late teens/twenties. 息子は10/20代前/後半です

I have a grown-up son. 私には成人した息子がいます

息子：(the) middle child/second oldest son

娘：my daughter/my youngest child/my baby/little baby/little one/my girl

My youngest kid, daughter, is taking ballet lessons on the recommendation of her mother. but I'm not a big fun of that.