I too have had his happen to members of a family, however, I viewed it the other way and it was fun having immortal sims. For me it started when I sent one of a couple on holiday (World Adventure). On return home his partner (who had stayed at home), had aged from young adult to elder but had become unageing. I swapped the pair and sent the other one on holiday and the same thing happened again, (The one who stayed home became older and unageing). So now I have both sims with the immortal gift/curse but aged. As it so happened I had also some potent youth potion of which both my sims partook.
I have this family saved and sometimes I bring them out of retirement.
However, to your original question is there any way to reverse the syndrome. I don't think so. I have tried all of the suggestion given me by the gurus and as I have already state I have saved the family and placed them in new game plays but they are still unageing. However, they can still dye from Mishap.
Things I have tried without success.
Deleted all the cashe files suggested. (Each time I quit the game).
Evicted them as a pair and placed on a new lot.
Split the family and evicted and place each one separately on a new lot, (This I did not like as they are a loving couple).
Saved to the bin, Quit, restarted and then replace on a new lot (Here they lost all of their friends and out of house relationships but they where able to return to their jobs without loosing rank).
Placed the bin-save into a new game.
However. Both of my sims had in their inventories the death flower and when one of them drowned he gave the reaper the flower and was restored to life. He was also restored to aging. I must admit that I did not try this on the other sim and did not save the the game.
