Does deuterated water slow down the overall metabolism of a cell?
Yes it would, by a few percent. It may or may not be a goal worth pursuing, but there is more to it.

Different reactions would be slowed down to a different extent. Tiny as they are, these discrepancies suffice to disrupt the delicate biochemical machinery of the living cell. No life form more advanced than bacteria is OK with that. Deuterated water in high concentrations is unpleasant for plants and downright lethal for animals, including us.

So much for your slowed aging.