As someone who has taught sex education for 36 years to students age 11-18 I think I am a bit of an expert on this. I was also a member of the county PSHE Team (Personal, Social, Health Education) a big part of which was Sex Education.
I think all children should receive good sex education and that should be age appropriate.
In younger children of Primary School age this should be around life cycles of animals, relationships and changes that take place in the body.
In secondary school this progresses into the fundamentals of human reproduction and would cover topics such as menstruation, masturbation and relationships.
By fifteen such topics as contraception, STDs, sexting, respect, homosexuality, human sexuality, sexism, dealing with emotions, love, relationships, inappropriate sexuality, pornography, pregnancy and abortion would be appropriate.
17 and 18 year olds would have detailed discussions on such things as the rights and wrongs of abortion, pornography and date rape.
These would not be run as ordinary lessons but usually be in the form of class or small group discussions and processing of information. The teacher would be there to lead discussions, enable and provide correct information when required (ie. on what types of contraception are available and where to get them or what symptoms of STD are and where to go for help).
I do not think parents should have the right to remove students from these classes. That would infringe the rights of students.
Good sex education actually reduces sexual activity, reduces STD infections and reduces pregnancy rates. It reduces stress in students who worry about their feelings, whether they are normal, relationships, emotions etc. It promotes respect and makes people more able to withstand peer pressure. It empowers people and removes the stigmas.