Srimad Bhagavatam Canto 11, Chapter 17, Text 39

SB 11.17.39

grharthi sadrsim bharyam
 udvahed ajugupsitam
yaviyasim tu vayasa
 yam sa-varnam anu kramat
 
Translation: 
 
One who desires to establish family life should marry a wife of his own caste, who is beyond reproach and younger in age. If one desires to accept many wives he must marry them after the first marriage, and each wife should be of a successively lower caste.
 
Purport: 
 
As stated in the Vedic literature,
 
tisro varnanupurvyena
 dve tathaika yatha-kramam
brahmana-ksatriya-visam
 bharyah svah sudra-janmanah
 
The purport of this verse is that one’s first wife must always be sadrsim, or similar to oneself. In other words, an intellectual man should marry an intellectual wife, a heroic man should marry a heroic wife, a business minded man should marry a woman who can encourage him in such activities, and a sudra should marry a less intelligent woman. The wife must be beyond reproach in terms of her background and character and should always be younger than oneself, ideally five to ten years younger. If one desires to marry a second wife, then, as stated in this verse by the word varnanupurvyena and in the verse spoken by Lord Krsna by the word anukramat, one must wait until the first marriage is established and then select a second wife from the next-lower caste. If one marries a third time the wife must be, again, from the next-lower caste. For example, a brahmana’s first wife will be a brahmani, his second wife will be from the ksatriya community, his third wife from the vaisya community and fourth wife from the sudra community. A ksatriya may first marry a ksatriya lady and then vaisya and sudra ladies. A vaisya can accept wives from two classes, and a sudra will accept a wife only from the sudra class. By this progression of marriages there will be relative peace in the family. These Vedic marriage injunctions, as mentioned in the previous verse, are especially for those who are not pure devotees of the Lord.
Srimad Bhagavatam Canto 11, Chapter 17, Text 38
Srimad Bhagavatam Canto 11, Chapter 17, Text 40