Though I know it's completely
unrealistic, I want to say 17 or 18 minimum, possibly higher for some, before they start having sexual relations. It's not just about the maturity and responsibility of the individual, though that is important. It's also about what Airen said. No amount of protection, however responsible the decision was, is 100% guarantee to prevent pregnancy and STDs. So even that mature 16 year old who waited till she was in a trustworthy relationship, who had been honest with her parents, who had made safe decisions to use protection and discuss it with her partner, could get pregnant or contract an incurable STD all before graduating high school.
I'm not a mom yet but I already think like one and it would break my heart. I have a very good friend who got pregnant in high school and she struggled so hard to fix her life after it. She has two beautiful children now and is a wonderful person but she wouldn't wish that on her children ever, ever.
But that's just about physically having sex. My opinion on how old the average
person probably needs to be in order to have a healthy, stable, emotionally-invested, mature relationship isn't until the 20s. I use myself as an example. I was a damn mature teenager. I was almost born from the womb as a mini-adult. I lectured adults on smoking, drinking, promiscuity, keeping their minds open, the importance of education, etc. at an early age. But even I wasn't mentally and emotionally "adult" until my early college years. Maturity in a child is not equal to the maturity of an adult that has lived more life and allowed time to mature their brain, settle their hormones, and give them life experiences.
/end wall of text