There is many different strains of HPV, the likelihood of already have contracted them all is small. It will still protect you against strains you don't have. It also protects against genital warts. The vaccionation program targets young girls because thats the most efficient time to take it and has highest benefit/cost. You will still reap benefits of taking it later.
I dont see any reason not to take if you get it for free and you are planning to be sexually active with multiple different partners.
I dont see any reason not to take if you get it for free and you are planning to be sexually active with multiple different partners.