So-called late bloomers, those who delayed dating until their late teens, accounted for 25%.
Late-starting daters, while also out of step with peers, appeared to have no apparent social or emotional difficulties.
The study, by researchers at York University in Toronto, used data collected from 1996 to 2003 on 698 students from 12 local schools.
The students, whose average age at the start of the study was 11.8 years, reported their romantic activities on yearly questionnaires.
Separate questionnaires assessed personality traits, such as shyness, depression, and aggressive or delinquent behaviors.
The early-starting group began dating at the age of 11.6 years, on average, compared with 12.9 years for on-time teens and 14.9 years for late bloomers.
A majority of students, 55%, were classed as on-time teens whose romantic activity gradually matured during adolescence.
Early starters, who began a consistent pattern of dating and intimate relationships between 10 and 12, comprised 20% of the participants.
Boys and girls who start dating at a young age are disrupting the typical pattern of teenage romantic development and may have more school and behavioral problems than their peers, suggests a Canadian study to be published in the December issue of the Journal of Adolescence.
Entering into intimate relationships too early can leave teens ill-prepared to handle typical problems couples face and without the support of peers at the same stage of romantic development, researchers said.