I will assume sister - may Allaah have mercy on you - that by youth you intend a shaabb. If that is correct, then you should know that many consider that to be one who has attained puberty but has not yet reached ar-Rujoolah (manhood). Others say it is the completion of thirty years of age or (some say) from sixteen years old to thirty-two years of age.
After that, a person is called kahl (or kuhoolah) or middle-aged. The age period preceeding kuhoolah is between thirty and forty.
From there he goes to shaikh which is from fifty (or fifty-one) to the end of his life or (as some say) eighty. May Allaah give all of you long lives on His obedience.
If I am not mistaken then there is a hadeeth which I am sure we covered in abu Talha's duroos of Silsilah ahaadeeth as saheeheh with the meaning of what was mentioned by the sister or brother. I will try and find it and post it ASAP In shaa Allaah.