Unfortunately, slowing down cellular activity will only slow down the organism as a whole. If you wanted to alter ageing by manipulating gene it would be necessary to do so to the genes (assuming they exist) related to ageing. Slowing down growth hormones would only slow down growth.
To maintain a person at an almost constant age would require major manipulations of their cellular and biomolecular processes. However, none of this is by no means certain. Much ageing is simply due to wear and tear on parts of the body or are the result of chance outcomes of metabolic processes.
Some degenerative diseases are gene-base, but definitely not all, and they might eventually be fixed. Tumours are the result of complex combinations of a person's genetic makeup and environmental factors.
Decelerating various cellular or biochemical activities will not solve any of these problems or producing de facto immortality. The trouble is ageing, cancers and biological pathways to immortality are vastly more complicated and difficult than that.
PS: If it's any consolation, you are already semi-immortality. You will live on through your children.
PPS: You do need to forget about the survival of your personality post-mortem though to benefit from your semi-immortality.