I think it depends on the skills and abilities of the person, not the exact age. Look carefully at every person. I have been working with a number of customers and relatives on computers for many years... this is just my thoughts... YMMV. I don't have an "answer", but some things to think about below:
Somewhere as you age it gets harder to "get" significant changes, including new technology. If a laptop or desktop is more familiar, then this may be better. If you have to change anyway (like XP to windows 7), then the tablet may be no more difficult than learning windows 7 quirks. Be careful, iPad is easier than most others I have looked at. Note that I have had great difficulty getting people (many much younger than 92!) to give up Outlook Express, which is not available with windows 7. And no, windows live is not a good match, it is significantly different. Users who could e-mail photos fine in OE can't get pictures to attach in Windows Live.
Some elderly have more motor control with their hands than others. A tablet or any
senior friendly phone requires good (not perfect) control for gestures, etc. Generally more control than a keyboard. If motor control is good, then a tablet or a
big screen phone for seniors are fine, if not it *may* be an issue. I have relatives that have issues with motor control, and the iPhone is somewhat harder to use because of that.