Romeo, Juliet and Renaissance Urban Demographics explains why gangs of young men rampaged through Renaissance Italian cities:
Based on very detailed information available from the Catasto, a combination tax assessment and census conducted in 1427, Professor Herlihy points out that the sex ratio between Florentine men and women aged 18 to 32 was 132:100. This is the kind of oversupply of men and undersupply of women that is normally found only in frontier towns. In the case of Renaisance Florence, this sex imbalance was the result of two causes. The first was a continuing immigration of ambitious young men from the countryside. The second was the tendency of Italian urban families to ship any unmarried daughters off to convents by the age of 15 or so, where they wouldn’t need expensive dowries and were apparently beyond the reach of the census takers.Added to the sex-ratio imbalance was the fact that urban males couldn?t get married until they could support a family. (This was the opposite of the situation in the Italian countryside, where you couldn?t start an independent life as a farmer without a hard-working wife.) And it took a long time to get established enough for a city-guy to contemplate marriage, if he ever made it to the altar at all. In Florence in 1427, only a quarter of men between the ages of 18 and 32 were either married or widowed. Married men didn’t make up the majority of the masculine population until they reached their middle 30s. The average Florentine child was born to a 40-year-old father.