The proper time and age of marrying is when the individual reaches sexual as well as mental maturity. Mental maturity may mean the capability of establishing a cordial family life and the ability to fulfill rights of family members.
The need of a spouse and family is a natural and instinctive need which Allah through His Wisdom has placed in human beings and is awakened at its particular time and season, and makes its demand. If it is answered on time and its requirement fulfilled, it traverses its natural course and makes the person perfect. If it is delayed or answered in an incorrect and unnatural mode, it deviates from its natural course, and insurges and rebels, and not only becomes corrupt itself, but also corrupts the man.
Who is eligible to marry?
For man to become eligible for taking a woman's hand in marriage, Islam has several recommendations. According to Islamic laws, when a boy attains the age of fifteen, or becomes sexually potent, he is baligh, and has attained puberty. But this is not enough for entering into a contract of marriage.
Apart from the laws related to puberty, there is a concept of Rushd which can be translated as 'capability of a sensible conduct' or maturity. A husband has to be Rashid and a wife Rashidah; so that the responsibilities of married life are sensibly discharged.
Books of Islamic law may be referred to for exact details on physical and mental maturity.
Recommendation for Early Marriage
Islam highly recommends an early marriage. Even those who feel they would not be able to bear the expenses of family are urged to repose faith in Allah, as He is the Giver of Sustenance (Rizq), and go for an early marriage.
 Youth and Spouse Selection, Ali Akbar Mazaheri, Ansariyan Publication, p. 34