Commercially gilts are usually selected for breeding at five to six months of age. The selected gilts are reared to weigh between 120 and 130 kg at seven and a half to eight months of age when they are ready to be served by a boar for the first time.
Gilts have to be in a good condition to produce large litters (eight to 10 or more healthy piglets) and should not be too fat when they are ready for mating. Therefore, they should be fed about 2 kg of meal per day from the time of selection until a boar serves them at the age of eight months. This will also ensure that not too much fat is lost during the suckling period and that they are in a good condition after weaning their first litter.
You may want to leave your gilts till they are older depending on the size of the boar that you have or are hiring.
My gilts were about 8 months old when they were served by the boar for the first time.