Urge for sex is a biological process observed in boys as well as girls and depends largely upon socioeconomic background, natural circumstances and individual’s sexual development.
It is perfectly normal to get attracted to a classmate, film star, teacher, a relative or a complete stranger. It is often an uncontrollable feeling when it first happens.
Since it does not establish love for that person, one gets adjusted to such feelings with passage of time as new emotions gradually develop.
Boys experience strong urge for sex at younger age and get aroused even by erotic pictures or films or by imagining sexual fantasies.
In Girls sexual urge is usually strongest in thirties and forties. They get attracted on emotional grounds and their sexual arousal in most cases is only after being caressed.
Not being interested in sex is also perfectly normal in young boys and girls. At the appropriate time, when the hormone level increases, the awareness of sexuality is felt.
Due to the involvement with studies, sports etc., some do not recognize sexual development till it becomes more apparent.
One must consult a doctor without any hesitation to clear any doubts and worries.