If by "loose" you mean promiscuous... lax upbringing, parents who had multiple partners, too-strict parents the child rebels against as they grow up, nonconsensual sex at an early age, a desire to "please" or become "popular" with the opposite sex (or sometimes the same sex), or a mix of these factors.
peoples sex desire deminish at different times. Some people don't finish all the way to their death.
may be no desire for sex
a person's sex drive or the desire to have sex.
its mostly age and it can also be a company or desire for companion There is usually something else behind it that they try to block out by having sex. Therapy helps with that.
Masturbating yes, and having sex with others depends on age. Most kids in USA and Europe loose their virginity at 15-17yo.
Find a hobby... Yoga or A type of meditative sport can rid of your desire, Or do a sport or activity that you like and you know will keep you busy.
By either masturbating or having sex.
Yes the age does play a big part, the older you get you loose intrest in sex and so also in getting a climax.
Libido is the term for sexual desire but other terms can be used: sex drive, passion, sexual desire, sexual appetite, sexual urge, erotic desire, sex instinct, or the hots (informal).