There is no age when you SHOULD start having sex. Some people never have sex and live just fine. I recommend holding off on teenage sex as long as possible simply because there are so many potential, life-altering consequences of having sex.
I recently read that there were over 400,000 teenage pregnancies in 2009. My goodness, talk about a game changer! Single teenage parents are overwhelmingly at the bottom of the economic scale struggling with living costs of trying to raise a child, which is hampered by a limited education which is exacerbated by limited employment opportunities.
The second consequence of being sexually active is of contracting a sexually transmitted infection which you may or may not be able to fully recover from. Teenagers are within some of the highest risk groups for contracting an STI.
The last issue is that teens tie up a lot of their emotional selves into the act of sex. Boys will often promise love in exchange for sex, while girls will often exchange sex for the promise of love. Neither situation is mature and certainly not healthy when the relationship falls apart. Understand that your teenage relationship is statistically-speaking unlikely to last the test of time. Are you ok with knowing that your partner may move on or are you expecting that your relationship will last forever? Are you starting a sexual relationship to try and win someone, or hold on to them? Sex is never a long-term glue for a relationship and neither is a child.
So, in answer to your question, I would say that you are ready to start having sex when you are mature enough to take the proper precautions for your own health and emotional well-being. Ensure you have and use a reliable birth control at all times. Boys, just because your partner is on birth control doesn't mean that you are off the hook. Plenty of girls have become pregnant while taking birth control. Protect yourself by always using a condom. This not only lessens your chances of becoming a teenage father, but you are further protected from most STI's. Girls, the same goes for you. Not only should you be looking after birth control for yourself, but also insist your partner wears a condom for the same reasons. Partners who double up on their protection nearly eliminate the risks of both pregnancy and disease.
Finally, before becoming sexually active, get vaccinated for HPV (Human papilloma virus) The younger you start having sex, the greater your risks of coming in contact with HPV. HPV can cause genital warts and some kinds of cancers. Vaccination will protect from most but not all types of HPV. Limiting the number of your sexual partners also helps reduce your risk of disease. Having sex is not for children. Only you can decide when you are ready to have sex, but if you're prepared to take some precautions prior to having sex, you will know that you are mature enough to make that decision.