When do Genetic Engineering make changes?

Changes at conception
Genetic engineering is most easily accomplished by making changes just after the egg and sperm have melded but before first division. In this way, the gene will be expressed throughout and will affect the recipients children, grandchildren, and all subsequent generations. Germline engineering is controversial because there is insufficient knowledge about DNA expression to accurately judge what result these changes will have.

Changes after birth
As of now, this is likely to take the form of gene therapy. This would not be hereditary unless the sex cells are engineered.