It depends entirely on the individual ! Puberty in girls can happen as young as 8 or 9 years old - or as late as 20. As for being 'sexually active' that is solely the responsibility of the girl.
If you have reached puberty, have had your first period, and are
sexually active, then yes. However, you should not be sexually
active at this age and much less be preoccupied about possible
pregnancy.
You dont think you will or want to get married so when is a good age to have the choice of becoming sexually active I think 18 is a reasonable age but im not sure.?
Age of consent is most often 18, but becoming sexually active
with a partner you love and trust is worth waiting for.