Around 15 i think . For what i know now a lot of young start dating at this age and the result seems to be good . Around 15 a lot of boys start to think about having a girlfriend ( cannot talk about girl desires ) and seems able to have a relationship ( including the sexual part ) .
Of course it is a very delicate thing but if you say to your boy/girl that they have to wait 18 , some will have relationship or/and sexual relations without saying it to you ( i think in most/all cases you ll not know about sexual relations and it is a good thing unless the girl speaks about it with her mother , some girls do it ) .
Others , more shy will not have a relationship , and i am not sure it is a good thing ( shyness is not a good thing , but you cannot say to boy/girls not to be shy . It is not a thing you can teach . These will have various futures ) . Even if a relationship at 15 can fail and last few weeks/few days , i think it is a good think to make young grown up ( and in long term it 'll motivate them because at 18 it is not the time to date for the first time but the time after first dating to make some life projects , you have to start choosing studies/your future ) .
Do as you want but try to not broke your son/daughter or push them more than necessary ( shy or not , fast or slow ) .