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.
So-called late bloomers, those who delayed dating until their late teens, accounted for 25%.
Interest in the opposite sex usually begins at puberty and gradually evolves into casual interactions, followed by group-based dating by the midteens, previous research has shown.
Exclusive partnerships are formed mostly later in adolescence.
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.
These experiences can increase the risk of unsafe sexual activity, alcohol use and delinquent behaviors, the study says.