Is it permissible for a girl to lie to people or to a suitor by reducing her age with the aim of getting married, and does sin result from that?

It is not permissible for a girl to lie about her age or anything else, for lying is forbidden except out of necessity. The Prophet, peace and blessings be upon him, permitted lying for reconciliation between people, in war, and for a man to speak to his wife and a woman to her husband.