Elder sims die as soon as they age up

I've had an issue for many years now where my elder sims die literally the second I age them up into elders. The only time this hasn't happened to me was when the sim had maxed the fitness skill and so had the perk where they lived a longer life. This has just happened to me again where I aged up 6 sims into elders at the same time and saved and left the game - as soon as I opened the game again within about 1 minute they had all died. This is seriously frustrating so help would be appreciated!