My parents taught me about sex when I was 4 years old just before my little sister was born, sex education should be a priority and start from a very early age. Children should learn the truth about where babies come from as soon they're old enough understand, it also makes it easier for them to tell someone if an adult is sexually abusing them.
However having said all of this there's a big differance between sex education and pornography, the latter isn't very educational and doesn't set a good example to children about safe sex or inform them of the consequences of unsafe sex.
