title says it all. From what i read on the wiki, toddlers and teenagers still die, but adults run away. On my 4U, my toddler ran away. Also, it took 2 1/2 days for it to die...
Why would they die? Tamas aren't just pets you take care of anymore, at least until adulthood, if you're not a good "friend" to them, they just get up and leave, since they live in TamaTown in these devices. It is an equivalent to death though.
Also, 4Us were created with adult women in mind, not children, so they're a bit easier to take care of. That's probably why it takes a bit longer for them to leave.
I haven't had a M!x die, but the manual seems to imply that they can, and when I was trying to get the horrible care adult form on my Anniversary, he spent a lot of time in his youth literally in an ambulance on the way to the hospital LITERALLY BEING CHASED BY THE SPIRIT OF DEATH. So... it can still be pretty stressful. X_X