In my current big save I will play with ageing on if I have pets in the home. Sounds mean that I don't get attached to the pets, but I love them having lots of cute pets.
But also I will keep it off if the sims themselves are not at a point where I am happy for them to age. I will age them up manually when ready. Elders tend to move into a 'retirement home' to live with their friends. :) (Ageing on so they will die when it is time.)
Every so often I will turn the ageing for unplayed/NPCs on to keep in line with mine too. :)
On a 'regular save' I will leave all ageing on for everyone.