Please explain if it is possible for a girl of 15 to get a mastopexy?

Possible but wait@ This is generally too to f and is generally not performed until completion of pubertal growth changes and is commonly deferred until completion of childbearing u less severe or performed for asymmetry.
It is but... There's much consider before having surgery to improve the appearance of your breasts. For one thing, what is your developmental process? If you are still developing or plan to have babies while young, surgery may be premature and have to be repeated. Are you emotionally/financially prepared for the stress of surgery? It's not impossible but is unusual at 15.